If the mechboxes are basically shooting the same (the M16 uses a full cylinder, where as the M4 uses a ported cylinder....but ignore that for now)...the M16 will deliver more accuracy at range. That means your effective range (you hit what you're aiming at) increases.
The differences aren't great though and the M4 length is 95% fine for whatever you're going to run into.
Batteries are batteries. Big batteries are better than mini batteries. LiPo's can pack big power in little packages.
Shooting with iron sights...the M16 will be more accurate as well because of the longer sight line between front and rear sights.
Make your selection based on what fits you best. I'm 5'6". I find the M16 just a little too long for me when I'm creeping/running through the brush. Tends to get hung up. I don't think I have short arms...but the full M16 stock makes me feel like I'm stretching a bit too much when shooting prone. Collapsing the M4 stock a bit is soooooo much more comfortable. Even an M4 is just a touch too long for me for CQB. When I'm shooting my secondary and go to one knee, I've got to take care not to pile drive the muzzle of the rifle into the floor. Longer than an M4 (i.e. FAL/M16/etc...) I'd be better off laying the rifle down to go to my secondary.
Best of luck,