I'm having a hard time thinking of a feature that beats having a battery that still has a decent charge at the end of a day of heavy use. Not having to think about your battery is a HUGE benefit.
From an overview of the M8's features, it looks like the main benefits are a higher resolution screen (which you'd be hard-pressed to notice), dual speakers (the MAXX is plenty loud, and it's not like phone speakers are going to sound truly good, just less crappy), and maybe a better camera? Everyone has their own priorities, so you've gotta decide for yourself, but for me the larger battery trumps those other features.