As a former S7 Edge owner and 6S Plus owner, I can tell you why I thought the iPhone performed better.
S7 Edge Hardware:
It has all the better hardware for sure. The screen is drop dead gorgeous. TouchWiz looks better than the outdate iOS UI as well. Quick charge is much better than lightning charge as well. No iTunes means cutting down on time spent with maintaining your phone. VR is great for the first few minutes. Water resistance is a must in modern phones these days. The S7E's camera is incredible. The Edge screens are useless, create hassle, and provide a dearth of accessories. The aesthetics aren't worth it. Heating issues are aplenty as well despite having "liquid cooling." A beefier battery, at least compared to the S6/S6E, does not help much because stand by battery life is awful. Notification LEDs and AODs are great combos nevertheless. The Snapdragon 820 is awful compared to the A9 and A10 processors. I left T-Mobile's S7/S7E before the updates to activate newer QAM LTE coverage. SD slots are fantastic to have as well. The fingerprint scanner isn't very good either considering Touch ID 2.0, Nexus Imprint, OnePlus, etc.
S7 Edge Software:
This is where everything goes south. Lag is essential to TouchWiz. Apps crash way too frequently for a flagship device. The resource management is so bad, that it feels like the phone runs with 1.5 GB of RAM rather than 4 GB of RAM. I spent way too much time disabling stuff to create a usable experience. Carriers disable features by default, thus inhibiting the user experience. The Whitepages spam feature isn't available on T-Mobile's ROM. Wi-Fi Calling isn't even baked into the OS, which makes buying an international Exynos model counterintuitive. The Samsung bloat is awful as well. Who programs an app drawer with Microsoft apps nobody uses? Why even sign the contract for that stuff in the beginning, Sammy? Then the app drawer needs to be reorganized from A-Z every so often too. Forget timely updates as well. Apps are also behind because of fragmentation, making it tougher for developers. Multi-window is only available for apps Samsung chooses. Despite "slimming down" the UI, making a more streamlined experience, etc, Samsung's positives are hardly noticeable because of all the issues. Smart Alert, the Edge glowing for people calling, etc are hardly noticeable.
iPhone 6S Plus Hardware:
The price tag does not warrant what Apple included. 2GB is a welcomed addition, but paltry at best. The 5.5" screen is only at 1080p and does not take advantage of the real-estate. A 3D Touch screen became more useful in iOS 10. The A9 processor is great, so great that it runs circles around the Snapdragon 820. The camera is solid, but it should not be the only 2015 flagship iPhone with OIS at the time it was released. The battery is big enough to give a solid experience without having to charge much. Lightning is an outdated standard. The minimal upgrade didn't make getting the 6S Plus worth it for me, but I was stuck with it after investing so much $ into a purchase. The headphone jack is a welcomed feature, especially since my iPhone 7 Plus doesn't have one. No back button and notification LED does suck from time to time. No cool features turns people off, especially when people like VR (albeit the gimmicky feature impression I get from it). Always on Siri hands free doesn't help much because she is so antiquated. Google Now and Google Assistant knock her down to the likes of S-Voice. The phone feels too bulky as well. It fell out of my pocket too much as well. The same thing with my 7 Plus. It does not have all the LTE features either, so it feels a little slower from time to time. You expect more out of a then $750 phone.
iPhone 6S Plus Software:
This is where iOS' reputation presents itself, especially if you want a no frills experience. 3D Touch, with iOS 9, was pretty negligible. With iOS 10? It becomes a bigger staple with your phone usage. You can finally disable Apple's trash first party apps. FaceTime & iMessage are the kings of instant real time communication. With iOS 10's new messaging features and emojis, it's hard not to appreciate RCS communication. Widgets present themselves in a nice organized fashion. There are less hiccups, but there will be moments when the phone can crash, albeit rarely. iOS provides timely updates, legacy support, and no carrier interference. I can get the newest apps first because of no fragmentation. 2GB of RAM means no app refreshing and tons of stuff can run in the background without issues because iOS' memory management is top notch. It's still missing some basic features that Android provides nevertheless: predictive dialing, an app drawer, UI customization, seamless ringtone changing, file system, etc. The Wall Garden is what Apple provides. The phone just sticks to doing the important stuff right and does not want to be your flashes in pans.
Overall, I used my iPhone 6S Plus more than my Galaxy. I had an opportunity to sell my iPhone 6S Plus or the S7 Edge during the first month the S7E came out, but I sold my S7E instead. When I got another S7E, I took both phones on a trip to Canada. I used the iPhone to communicate more because of iMessage and a more user friendly experience. I tried to go with the S7 to save $ by ditching the 6S Plus, but the plan backfired because of all the things I missed and the constant issues I experience with deceitful sellers, overheating, and lag. I eventually moved to the iPhone 7 Plus and a Nexus 5X combo. Both work fine for me, but I am always looking to ditch the 5X for a better upgrade without spending too much $ in the difference between buying another device and selling the 5X. I am pleased with iOS, but think it needs a revision for the 10th anniversary coming up later this year. I am rooting for Samsung though.
If Samsung makes a great hardware experience, guts the carriers from bloating their devices, offer timely updates/legacy support, provides wifi calling into Touchwiz, and provides no lag, I might be back on the S8. I am not rushing into a S8 purchase, especially when I learned my lesson from the S7. I would recommend an iPhone 6S Plus over the S7 Edge as well.
S7 Edge Hardware:
It has all the better hardware for sure. The screen is drop dead gorgeous. TouchWiz looks better than the outdate iOS UI as well. Quick charge is much better than lightning charge as well. No iTunes means cutting down on time spent with maintaining your phone. VR is great for the first few minutes. Water resistance is a must in modern phones these days. The S7E's camera is incredible. The Edge screens are useless, create hassle, and provide a dearth of accessories. The aesthetics aren't worth it. Heating issues are aplenty as well despite having "liquid cooling." A beefier battery, at least compared to the S6/S6E, does not help much because stand by battery life is awful. Notification LEDs and AODs are great combos nevertheless. The Snapdragon 820 is awful compared to the A9 and A10 processors. I left T-Mobile's S7/S7E before the updates to activate newer QAM LTE coverage. SD slots are fantastic to have as well. The fingerprint scanner isn't very good either considering Touch ID 2.0, Nexus Imprint, OnePlus, etc.
S7 Edge Software:
This is where everything goes south. Lag is essential to TouchWiz. Apps crash way too frequently for a flagship device. The resource management is so bad, that it feels like the phone runs with 1.5 GB of RAM rather than 4 GB of RAM. I spent way too much time disabling stuff to create a usable experience. Carriers disable features by default, thus inhibiting the user experience. The Whitepages spam feature isn't available on T-Mobile's ROM. Wi-Fi Calling isn't even baked into the OS, which makes buying an international Exynos model counterintuitive. The Samsung bloat is awful as well. Who programs an app drawer with Microsoft apps nobody uses? Why even sign the contract for that stuff in the beginning, Sammy? Then the app drawer needs to be reorganized from A-Z every so often too. Forget timely updates as well. Apps are also behind because of fragmentation, making it tougher for developers. Multi-window is only available for apps Samsung chooses. Despite "slimming down" the UI, making a more streamlined experience, etc, Samsung's positives are hardly noticeable because of all the issues. Smart Alert, the Edge glowing for people calling, etc are hardly noticeable.
iPhone 6S Plus Hardware:
The price tag does not warrant what Apple included. 2GB is a welcomed addition, but paltry at best. The 5.5" screen is only at 1080p and does not take advantage of the real-estate. A 3D Touch screen became more useful in iOS 10. The A9 processor is great, so great that it runs circles around the Snapdragon 820. The camera is solid, but it should not be the only 2015 flagship iPhone with OIS at the time it was released. The battery is big enough to give a solid experience without having to charge much. Lightning is an outdated standard. The minimal upgrade didn't make getting the 6S Plus worth it for me, but I was stuck with it after investing so much $ into a purchase. The headphone jack is a welcomed feature, especially since my iPhone 7 Plus doesn't have one. No back button and notification LED does suck from time to time. No cool features turns people off, especially when people like VR (albeit the gimmicky feature impression I get from it). Always on Siri hands free doesn't help much because she is so antiquated. Google Now and Google Assistant knock her down to the likes of S-Voice. The phone feels too bulky as well. It fell out of my pocket too much as well. The same thing with my 7 Plus. It does not have all the LTE features either, so it feels a little slower from time to time. You expect more out of a then $750 phone.
iPhone 6S Plus Software:
This is where iOS' reputation presents itself, especially if you want a no frills experience. 3D Touch, with iOS 9, was pretty negligible. With iOS 10? It becomes a bigger staple with your phone usage. You can finally disable Apple's trash first party apps. FaceTime & iMessage are the kings of instant real time communication. With iOS 10's new messaging features and emojis, it's hard not to appreciate RCS communication. Widgets present themselves in a nice organized fashion. There are less hiccups, but there will be moments when the phone can crash, albeit rarely. iOS provides timely updates, legacy support, and no carrier interference. I can get the newest apps first because of no fragmentation. 2GB of RAM means no app refreshing and tons of stuff can run in the background without issues because iOS' memory management is top notch. It's still missing some basic features that Android provides nevertheless: predictive dialing, an app drawer, UI customization, seamless ringtone changing, file system, etc. The Wall Garden is what Apple provides. The phone just sticks to doing the important stuff right and does not want to be your flashes in pans.
Overall, I used my iPhone 6S Plus more than my Galaxy. I had an opportunity to sell my iPhone 6S Plus or the S7 Edge during the first month the S7E came out, but I sold my S7E instead. When I got another S7E, I took both phones on a trip to Canada. I used the iPhone to communicate more because of iMessage and a more user friendly experience. I tried to go with the S7 to save $ by ditching the 6S Plus, but the plan backfired because of all the things I missed and the constant issues I experience with deceitful sellers, overheating, and lag. I eventually moved to the iPhone 7 Plus and a Nexus 5X combo. Both work fine for me, but I am always looking to ditch the 5X for a better upgrade without spending too much $ in the difference between buying another device and selling the 5X. I am pleased with iOS, but think it needs a revision for the 10th anniversary coming up later this year. I am rooting for Samsung though.
If Samsung makes a great hardware experience, guts the carriers from bloating their devices, offer timely updates/legacy support, provides wifi calling into Touchwiz, and provides no lag, I might be back on the S8. I am not rushing into a S8 purchase, especially when I learned my lesson from the S7. I would recommend an iPhone 6S Plus over the S7 Edge as well.