It looks like you're new here. If you want to get involved, click one of these buttons!
It's not because TERA is any better of a game or can provide any more entertainment. It's not because of gameplay or graphics. It's not because of a subscription versus cash shop, or BAMs or quest hubs or whatever else you want to throw in here.
It's because TERA is already out. I have enjoyed both TERA and GW2 so far. However, I know I can log on to TERA right now, yet I have no idea when I'll get to play GW2 again. I'm not saying don't buy GW2 when it releases or that it will provide any less entertainment than TERA. I AM saying, would you rather have fun now and more fun later or just fun later?
If the box price + sub is an issue for you, then maybe it is a better idea to wait if you believe you will prefer GW2 or if the pricing model will work better for you. Otherwise, if the cost isn't going to hurt, I would recommend at least giving it a try. Can't go wrong with two fun games instead of one fun game.