I agree with you that hardware wise the S6 is a winner. However, the software experience is just as important, and from my point of view that is where Samsung has failed.
I have yet to use a TouchWiz device that I liked, and I was really hoping they'd revitalize the UI with the S6. While I'm very interested in the S6, if I see even a hint of lag or stutter in the UI in the released version -- and I have seen it already in videos today -- I'll stick with the iPhone for another generation just because of the software. If I could get it with stock Android, it would be a no-question purchase for me. I don't root or do custom ROMS, so I rely on the released software to provide a smooth experience. The S4 failed me and the S5 failed me. I sincerely hope they've got lag and stutter worked out with the S6, as they claim they have.
I don't have any real issue with Touchwiz, and there are elements of tasking in touchwiz that make iOS seem unnecessarily arduous. While iOS most excels in presentation simplicity, iOS is not simple. It's actually overly complicated for many simple tasks.
Touchwiz is resource hungry. The S4 was a laggy turd, but my S5 and Note 4 are quite smooth. I had an iPhone 6 for 13 days and returned it because the experience kinda sucked, and the crappy screen and soft camera imagery left me cold. 1GB of RAM and weak battery life are a huge problem.
I want to return to Apple and get the smaller iPhone 6 (I'm getting big screen fatigue from my Note 4 after 3 months of ownership), but I won't do it for merely average Apple products.
I want a better screen, I want no concerns about bending, I want more RAM, I want more battery life, I want all those nifty iOS 8 features to actually be supported, I want a better camera, I want 32GB to be standard.
I want Apple to create a Galaxy S6 caliber device with iOS, because pretty much nothing about the iPhone 6 is impressive to me.