I used the M8 for 6-7 months prior to getting the 6 plus and rarely had any app crashes on it either. Very stable. I honestly love both platforms - I'm not trying to troll here, I really like the 6S plus so far. I'm just frustrated that every iPhone I've owned has been unstable.Interesting. I dont dispute your experience, I can just honestly say mine is very different.
Ive gone Galaxy s1 (Samsung fascinate) -> iPhone4s-> Galaxy s4 -> iphone 5 -> htc one m8 -> iPhone 6 -> note 4 -> iPhone 6s Plus.
In every case, I got to android as I like the flexibility of the platform, yet always have app crashes/non responsiveness that drives me crazy and brings me back to iOS, which is much more stable though locked Down.