I play both, and they each have there strengths and weakness. I think SWTOR wins for me, mostly because of the level of storytelling and immersion. The lore in WoW is good, but very rarely dose the game ever do it justice, and never do they provide a real level of immersion with the world of Azeroth or the player character.
I really don't think one is better or worse, but each game does something a little better than the other. WoW has had nearly a decade of time for polish and lots of money to add new features. On the other hand, it's a very old game and it's showing its age.
The reason why I don't play WoW much anymore is just because I've become tired of it.