Its hard for me to say which is better since right now I don't like WoW anymore, but I think I agree with Tage if they had both come out at the same time I'd rather play WoW. But now since I've been playing WoW for almost a year I'd rather play this, hitting 60 killed WoW's fun for me there's nothign enjoyable to do after that. And I don't consider constantly running instances to try and get slightly better equipment fun, also I don't (and have never) care for MMORPG's PvP systems or thought that genral MMORPG combat was fun for PvP.
I'm sure I won't care much for GW's PvP either, but at least going into it my chances are based off my skill selection, playing skill and group rather then my class.
I think GW's top selling point over WoW is that it's more causal of a game (which is hard since WoW is very casual until you hit 60), and has an actual story that you can play. WoW is way more solo friendly (I soloed about 3/4 of the non-instance game) and is much more indepth world wise, but WoW is a MMORPG and GW isn't.