If playing with veteran characters is not a concern of yours I would suggest Balmung. While Gilgamesh has a strong and growing community, Balmung has been established as the RP server for far longer and because of that it is more likely to have a larger community both at launch and after.
If you are concerned about playing with characters who have been around for 1 or 2 years then I'd suggest you try out Gilgamesh and see if it fits. There are a few other servers as well that will not be as populated as these but will still have RP on them, I think they were Behemoth and Leviathan. Ultimately I suggest you figure out what your priorities are when you get in game and base your decision on that.
If you are concerned about playing with characters who have been around for 1 or 2 years then I'd suggest you try out Gilgamesh and see if it fits. There are a few other servers as well that will not be as populated as these but will still have RP on them, I think they were Behemoth and Leviathan. Ultimately I suggest you figure out what your priorities are when you get in game and base your decision on that.