So I have both, and they each have their perks...
I6 had the better camera, which matters to me as I take alot of snaps. I also like some apps better on iOS than android, especially some of the fitness ones. The screen is nice, even though not as high a ppi as m8. It also is less noticeable in my pocket, I always am very aware when i have the m8 in my pocket. Apple maps suck, but all the Google Apps I love I can use on it. I also like that i can use headphones to control volume, not just start/stop/talk. IOS is a tad boring, but my m8 is set up similar, I just hate how iOS puts all settings together, vs in the apps where it makes more sense.
M8 has fun camera tricks, but can't zoom in without awful pixelated. I hate Siri and Google now is light years better. The screen looks nicer, and is a bit bigger, but the onscreen keys are generally visible cutting into the screen. I don't customize much, beyojd keyboards, which ios8 now allows. I really like sense, it just feels modern and classy. Sound quality is great, and I love the feel of it, although it is bordering on too big for me, especially in jeans pockets.
I like them both, both do some things better than the other. When travelling, iphone6 is what I carry, due to camera.