Right now theirs minimal difference but the way it's supposed to work is:
In cultural PvP you can only attack members of the opposing race, you can still group and converse with anyone normally though.
In FFA PvP you can attack anyone of any level of any culture of any class at any point and any place...it's a "clusterf*ck" so to say. You can still group and chat with anyone as well though.
Unfortunately the rules for the Cultural PvP don't work quite right at the moment so if you're grouped with, say, a Stygian, and someone of your race, Cimmerian, attacks them, you can't fight them back.
Also, the fee is 14.99 USD, you get your first month free with purchase of course.