[ujiro]

Best bet is to go with the standard M4 length, carbine length. If you are unsatisfied with the performance accuracy-wise, then you can always slap a tightbore barrel in, or even a silencer with an extended barrel underneath it.

But really it should come down to what you like the look of best.

Edit: but if you plan to do indoor at any point, a short one would be a better bet.