To me they are all the same, I played them all and there's really no noticeable difference between the 3 factions aside from the appearance of character factions and weaponry again to me. I've heard people say one faction has mroe powerful weapons but at the cost of accuracy, other faction are high accurate but not quite as powerful and one factions is supposedly 50/50 but during all my play time on all three factions I didn't notice any of these and it just felt the same.
I pulled this out of a thread in the beta forums
Originally posted by another user
Raven: Clean Cut, Tactical, Modeled after SWAT
+Rate of Fire
+Accuracy
-Damage
Valor: Tough, Grizzled Veterans from the U.S.
Balanced
SVER (Seryi Volk Executive Response): Middle Eastern, Band of people looking for a cause to fight for, has the kind of 'punkish' look
-Rate of Fire
-Accuracy
+Damage
There was a more in-depth thread regarding the factions but I couldn't find it amongst all the noob spam in the beta forums. Again I personally didn't notice a difference but apparently other do so take it for what you will. Btw, I haven't received the code yet and someone said they were supposed to send them either today on the 14th or starting today. In the beta forum I read they won't send them out until the 17th and onward which didn't make sense but I'll let you know asap. Once you get the key you'll be able to visit the beta forums and read on stuff for yourself.