Mainly its down to the weapons. Some have a higher fire rate but deal slightly less damage where as another will have more damage but less fire rate. As well as aiming and recoil etc.
Raven maps are easier to defend (I've been in Valor and currently in Raven) specially Acquisition, S.V.E.R. is where the 'hardcore' gamers were in the beta and most of them stayed in that faction for the game. Valor is kind of the mid point between the other two, there maps are neutral and their weapons too (not too powerful, yet with good fire rate).
I think it comes down to the looks and which faction you like the most, or where you have more friends at to play with.