This question is one-sided, there is no particularly good answer, these two are classic games, each with a lot of players.
And every game has his hot period, and World of Warcraft should be very popular in the next few months, but Final Fantasy XIV is not just outdated.
I bought FFXIV Gil at MMOAH last week and probably know that Final Fantasy XIV is still very popular. To compare it from all aspects, the best way is to Experience both games once and experience it.