That's a tough question. I play both and enjoy both so I would say both if you can afford that.
Imho, WoW is still one of the very best MMO on the market right now, but it can be very difficult to get into, especially right now. While people are complaining about the lack of content, there is a lot to do even right now, but it's totally up to the player and what he wants to achieve.
In the meantime, FFXIV is rather fresh. It's a very polished game with a solid game system. The approach they have with the raiding content is totally different. I wouldn't say better than WoW, just different. And it's often the case with several aspects of FFXIV.
The real strength of FFXIV for me is that it is a real Final Fantasy game, with everything you can expect from a game of the license (great story, music, aesthetic) and that epic feeling that WoW may have lost along the way.
But to be honest, I fear that in the long run, FFXIV is going to be spoiled by his community. While the community has to be praised right now for being for the most part helpful and nice, as soon as the people will get more knowledgeable about the game and the way it works, the game will suffer from it.
I just hope Square Enix will keep the level of secrecy they have right now.