I don't know what Cactuar is like, but I'm enjoying life on Faerie. It's a middle population server. I'm always making friends and have never felt like there was a shortage of other players in any aspect of the game. The only downside seems be that Aether data center is unstable since the patch... likely because Balmung on Gilgamesh are also on it.

Balmung is the unofficial RP server and is one of the big old servers. That combo draws so many people that even the apartments are sold out. Keep that in mind if you think of Balmung. It might be too big. Faerie has a little bit of RP. Not as much as I'd like, but I don't really focus on that anyway and I enjoy having an apartment with the prospect of a house not being unattainable. If you want Balmung or Gilgamesh, you'll probably have to use a paid character transfer, as those two are locked down.