Honestly, the impression I got from TERA Online (or at least the 20 minutes I got to play on my friend's computer) was as a korean attempted rip-off of Rift which tried to copy Guild Wars 2's combat system, albeit fairly unsuccessfully. I mean, this is obviously a somewhat blunt (and perhaps unfair?) comparison, but it is honestly the impression I got from the near-half hour of gameplay I allotted.
Guild Wars 2, on the other hand, seems to be trying new things and going in new directions, at least with the things which matter. The 'Golden Trinity' is no longer holding players back from having fun, you no longer have to grind or 'farm the best gear' just to be on-par with everyone else in PvP, and you don't have to wait half an hour to start a dungeon just because you're lacking a 'priest' class or a 'tank' class. I will admit I am biased, though, since I did already pre-purchase the game and was in the last weekend beta event (and it was awesome lol).
For me, Guild Wars 2 wins by default, just because they got rid of the necessity for the 'big three', and replaced them with three of their own. The difference is, any class and fill in any role, be it support, control, or damage.
Plus, Free-To-Play compared to a subscription is a really sweet bargaining chip, as well. lol