I'm switching from the HTC One M8 to a 6 Plus. However, I'm biased and won't provide a true Android-only opinion.
I've had every iPhone up until the 5S. I've also owned 3 different Android devices in between there as well - HTC Inspire 4G, HTC One M7, and HTC One M8.
I initially sold my 5 and went to the M8 for the larger screen. I love larger screens and that was the sole reason I switched. For the 3rd time, I'm going back to the iPhone. I'd guess this will be the last time I test the waters with Android. I kept giving Android the benefit of the doubt but there are too many little things I simply don't like. I like the customization, yet, after experiencing bugs a couple small bugs or things that just weren't as fluid and simple, I want iOS back... badly.
I've had the M8 for over 6 months now (since launch) and I'm dying to go back to iOS. I have an iPad Air (which I absolutely love) and an Apple TV. Having a whole ecosystem with Apple is simple. It makes my life easier (AirPlay, for example) and provides a smoothness with transitioning between devices - whether it be the huge benefit of iMessage (and the ease of being able to text from my iPad easily), to seemless backups, and overall (IMO) higher quality apps.
Again, I like Android a lot, it can do everything that iOS can do... and more. Yet, finding solutions to certain scenarios could take 4-5 steps to complete, whereas on iOS it would take 1 or 2 steps.