4GB is usefull for triple screen or 3d but 2gb is enough for any single screen resolution
It depends on the game you plays, in some NVIDIA win and on other it's AMD that's winning... Check benchmark of your favorite games and decide on that...
I would consider both depending on price but the FTW is surely cheaper and they're the same card, only memory option for 4gb vs 2gb but you can have the 4gb FTW so...
Based on price in my country, the FTW 2gb is almost 100$ cheaper so i would get that one...