If you play at a resolution much higher then 1920x1080p or use multiple monitors at 1080p then you should get the 4GB, if not then go with the 2GB. Most games won't break above 2GB, but recently some games like skyrim and BF3 have broken 2GB when playing at higher then 1080p rez. It really depends on what you have now and what you want for the future, if you think your gonna get a insane screen or use multiple monitors in the near future to play PC games then go with 4GB to future proof. If your gonna only stick with 1080p then go with 2gb, i highly doubt FFXIV will even use 2gb.
Here are some benchmarks comparing a 2GB vs 4GB of the same card, notice how for all the resolutions at 1080p or lower, the FPS is the same, for some games even at higher res the FPS is the same.
In regards to the FTW version, this really depends on the specific card so it would be more helpful if you could link the 2 cards your looking at.