get the M4, you can always carry an extra battery as a backup and you still have the flexibility of the retractable stock etc. longer barrels are usually better for accuracy, so it depends on the type of player you are, if you want to be a run 'n gun kinda player then the m4 might be better, if you are more of the marksman type then maybe the m16 might be better, in the end it comes down to personal preference
