Three things made my decision incredibly easy:
M8's awful camera, the inability to change the battery in the M8, and the lack of a physical home button on the M8.
I guess I also prefer TouchWiz to Sense, but if the phone was good enough I'd have made the switch. It wasn't, ergo easy decision.
I almost always switch out the battery when I get a new phone to an Anker one. The S5's is 2800mAh, but so far it's lasted about 54 hours on a single charge with constant Wi-Fi & 4G LTE connections (and about 10% left, without going into Ultimate Power Saving mode). I probably will hold off on replacing the battery until this one starts giving me problems