Is the HTC One M8 as close as it gets to the iPhone 6 and 6+?
I have both the HTC ONE (M8) and iP 6+
Here is my take for what it is worth:
1) HTC Camera lens (not sapphire) scratches easily. Common problem. There is a thin film that scratches and you have to send it to get it fixed. I know people who sent it 3-4 times. Others recommend sanding the film off by using toothpaste.
This cause photos to glare and have white spots. This is a pervasive problem I joke you not. This is not accounting for the dramatic photo-quality difference. The IP6 has a really good camera. Since I have a physical defect on my HTC, I don't use the camera at all.
2) SDCard is handicapped. KitKat (and further versions of Android in future) does not allow apps to go out of their sand-box. What does this mean? You can't use ES File explorer to move files around. Have a USB OTG Dongle? You can't copy files from your USB stick to your sdcard. Again, this is known and is just a matter of googling to know I am not kidding on this. I bought a 128GB sd card just to see it un-used due to the restrictions. Apps can only write to their little sandbox and it is stupid.
I have picts on my Google Drive. I download it. Gallery sees it. VSCOCAM has no access to it. I can't edit my jpeg I download off Google drive in VSCOCAM to upload to instagram. Again, those with HTC ONE M8. Try this exercise I just laid out. So this is part of the sandboxing restrictions Google has enacted.
3) UI elements grossly over-sized on the HTC. Toolbars, navbars, fonts are way too big. I have a 6Plus and I can see way more content. Also, since it has soft-virtual keys, web browsing real estate is taken up by the virtual soft keys. Emails, I see more lists of text. For example, Instagram, I see more content. 12 thumbnails vs 15-16 on the IP6. And Instagram isn't even optimized on the 6 plus yet!
Despite both having the exact same 1920x1080p canvas area, the IP 6Plus will always show more content on-screen due to the proportions of the UI.
HTC UI reminds me of folks who run disability assist mode on their PC who have their screen DPI multiplied by 2x, and run XTRA-Large window themes. If you have aging eyes, you may prefer the bigger buttons, text bar, shorter lists. I don't.
4) HTC has better sound card DAC. But that point is going to be moot because lightning 24bit/96k audio DACs will be coming to iOS very shortly. So if you have $500-800 headphones, you'll need a DAC/headphone amplifier. Both the HTC and iPhones due not have enough juice to power the higher end audiophile headphones. I rather have a small slimline lighting solution than a bulky USB-OTG-box and headphone amp.
5) HTC has extreme power saving mode. You can go 3 days if you just use it as a phone. IP6, 2 days but then again, I'm actually using the camera lot on the IP6; editing photos and editing video.
2 & 3 can be solved with rooting, third party launchers, new mail app, new email client, and a 3rd party web browser that gives you full screen. But, you will be relying on 3rd party solutions for the HTC. For the IP6, those things just work.
The biggest selling proposition of Android and SDcard is no longer there.
Everything else is personal preference. Oh, for the first time in 7 years, I have actual Group IPSEC VPN on Android. HTC bundled a 3rd party app that actually works. I had that on iOS for 7 years as a requirement to connect to multiple VPNs I needed. I went through a lot of flagship Android phone and the HTC was actually the first I could use for work.