In a game environment you're not going to notice a huge difference in accuracy between a stock M16 versus a stock M4. Add in upgrade components like a better hop-up and a tightbore barrel, however, and you're going to see an improvement. In my opinion, people who fuss over infinitesimal increases in accuracy or performance are missing the point of the sport.
If you prefer the look and feel of the M16, get the M16. If you prefer the look and feel of the M4, get the M4. Your choice of AEG is 80% personal preference. My team, for instance, runs with AR's so we can swap magazines in the field and do group purchases for upgrades and replacement parts.
Anyway, there's no rule around here that you can have only one gun... If you can't decide, buy them both.
IN OMNIA PARATUS