Guild Wars 2, for certain. And no, I won't take the easy path by saying "it's just personal taste".
Guild Wars 2 at the moment is a better game. It has better graphics, better PvP system, better levelling system, and innovates in so many ways that you can't count then. And it's FREE of that damned subscription fee.
WoW was an incredible masterpiece, absolutely game changing, when it was released. But now, in 2012, GW2 is quite simply a better game which costs you less money. The choice is a no brainer, really, unless you have personal reasons (i.e.: friends in WoW you really want to play with) which make you stay in MoP.