i prefer the rifle because of its backward weight, this comes really handy when mounting a Masterkey or m203. And changing the battery is faster because you only need to open a small hatch in the fixed stock, while you have to dissasemble the handguard on the carbine and this can cost you many precious seconds. Using the M4 Carbine with a RIS-Mount, makes it top-heavy, and you need an extern battery holder and this gives you additional weight, and therefore it gets really uncomfortable to play with IMO !
But if you just want to use an M4 without a Mount, the Carbine is your best choice, because of its light-weight!
And another reason why i prefer the Rifle....there aren`t so much running around with a rifle
Snake, if you are interested in playing, i can give you an link to an Airsoftclub near Graz, or you stop by us in Upper Austria, almost every Club has at least 1 ASG for rent !