I own and like both GW and WoW.
The scope of WoW is simply awe-inspiring, I readily admit that. The major hit it takes, in my opinion, is from a combination of the fee and the disgusting amount of time you need to devote to get anything interesting (I'd like a mount some time eariler than level 40 >_>). For those of us who are into post-secondary education, that kind of time requirement coupled with a fee is no good. However, the variety of areas, items, armour, etc. is dazzling. The classes are very well balanced, and the fact that the entire server is, to some extent, at war (horde VS alliance) is simply neat.
However, I find that the first few quests (well...not few - my character is at level 20 and I still have an issue with this) are horribly tedious. A person, for instance, could ask me to bring him the heads of 12 bears. Ok! I accept the quest and begin my hunt. I kill some 35 bears before I get the 12 heads that I need (apparently, some of these bears, despite their appearance, don't have heads). I'll return after my valiant efforts to receive my decent (not great) reward, only to get another quest from that same peasant/solider which involes something else from a bear's corpse. Frankly, there are a few too many quests like that, imo. I've never been a fan of those quests - they only generate tedium. However, there are a few quests that may, occasionally, balance that tedium out and are simply a lot of fun to tackle with a group. Plus, the scope of the Alliance-Horde conflict really adds a lot to the game - watching a particular PvE incident in Stormwind showed me that. WoW does, indeed, have more to offer than your typical MMO.
I also found a lot of good points in GW. I REALLY liked that GW's quests focused on moving you through a neat, fantasy story. When you performed a huge, story-affecting mission, you felt like a hero at the end. This varies from a lot of MMO's I've played where major quests involve you waiting for a hero to respawn after some higher-level tank killed him before you got there.
I liked, for the most part, the fact that all non-town areas are instanced. It prevents having to compete with annoying grinders/farmers. You do what you want in your space (unless, of course, you opt to bring a party along). I also liked the emphasis on strategy that I found in GW. The key to progressing through the story (ie: completing a mission) is a good group. Sometimes, that means a well-balanced group (a tank, a healer, a few mages), or that could mean a group that's made entirely of tanks. Not only that, but you need the group members to be attentive and open - people stuck in their ways will likely get you killed. It's a lot of fun to get the right group together and plan your way through the missions as you do them. That's not something I've really done in any other MMO. And, if you can't find a good group for some smaller quests, you can always hire henchmen (for free) to help out. However, enemies that pop out of the ground and the VERY large agro range make for some very tedius exploring in GW....and I mean very tedious at times.
Lastly, I really liked the sense of direction in GW. It's open, in some respects, but there is also a guiding story to follow that leads you through the game's nations. I, for whatever reason, don't always want that "go wherever you want" feeling. Sometimes, frankly, that's too open, and I'll likely end up wandering into a den of beyond-cap-level mosters that will slaughter me.
My recommendation? If you have the time/money, try both. I don't really compare WoW and GW often because I enjoy what each has to offer. A lot of people will condemn GW saying that they prefer WoW, but I think that people are not taking into account the fact that GW was not meant to be another WoW. It offers a completely different feel, and I don't think people should let it slip past simply because it isn't like every other MMO. At the same time, WoW isn't the typical MMO. They have tried, earnestly, to get rid of the grind (and it is present to a lesser extent than other MMO's), and the sheer scope of the game is a selling point in itself. The game is balanced, and the community has several helpful players in it.
Try both, heh.