If you liked the M8 so much, why not go for the M9?
Mostly because the M9 is so friggin' expensive
the m9 has overheating issues and its not much of an upgrade imo
That's kinda my main concern with the ASUS Zenfone 2 as well, because it has an Intel Atom processor inside of it.
I heard that the processor was great, the Zenfone has a power button at the top which might be bad and i heard the software was kinda eh