For me the Samsung stuff is still just not up to par software wise, and I can't put all of that on Android's back. The TouchWiz skin is STILL a lagfest after the first week or so, and it still is a bloatfest. No excuse for that in 2017.
Samsung makes killer hardware, but I've never used a Samsung phone that didn't lag. Not even the S7/S7 Edge or the now defunct Note 7. They still stutter and hiccup and it is just mind boggling. It proves specs don't matter when the OS and/or it's skin is so poorly optimized. Again, great hardware and great features, but sorry excuse for their software.
I've never had any hiccups or lag or crashes on an iPhone and I'm in my third one. 6 Plus, 6S Plus, and now 7 Plus. iOS 8 through 10.2 (though I've had experience with 6 and 7 on iPads and still nothing). They were all super fast and I've never had as much as a app crash or a respringboard or any lag or stutter or slowdowns after awhile. I realize it happens, but it happens MUCH less than a Samsung. I recently helped a friend set up her S7 Edge and it was a lagfest out of the box. It may only be a second or two at best, few seconds at worst but that builds up and there is no excuse for it in comparison to an iPhone, or even a Pixel/Nexus/Stock Android device. Even LG and HTC phones lag less than Sam's. They better step it up to compete this yr.
If you need the customization and feature set of Android, get a Pixel or Pixel XL.
If you like iOS, stay with an iPhone or iPhone Plus. I think you will be better served.