The factions will provide you with exclusive weapons, armour and other customisation options for your character and vehicles, but apart from this, not much else is known. I think that factions could link into your character development, with certain factions rewarding you with certain types of weapons. For example, for an aggressive player, Seven Seraphs could be a good choice as their (only at this point) leaked weapon is the Pocket Infinity sub-machine gun, which could help you in CQC. Whereas another faction such as New Monarchy could reward you with huge machine guns such as Super Good Advice (I assume Super Good Advice is new Monarchy because of the colour scheme) Just a thought :)