I've played both and I'd say WoW at this point. It is somewhat of a personal choice. I think FF11 could have been way better if it wasn't designed for a PS2. The interface was crap and they had a huge limit on textures it seemed, often repeating the same monster time and time again (most games do this, but FF11 brought it to an all tooo noticeable level).
FF11 Strengths:
- Nice economy implementation
- Fun battle setup with combination moves
- A lot of story for an mmorpg game
- Chocobos
Wow Strengths:
- Awesome interface / graphics
- Balanced warcraft universe with a ton of content
- More quests than you could ever really want
- Decent RVR system (decent is good for an mmorpg)