I'm slightly biased because I've been enjoying Guild Wars for a year now, but IMO it's the better game. I watch my GF play WoW (because she has a mac, and can't play GW on it), and the amount of frustration she gets at things like other players stealing the monsters she's trying to kill, spending 5 hours doing a raid so she can have a tiny chance of getting the armor she needs to be able to move on to the next raid, players from the other side purposefully walking into her attacks so they can then kill her, etc.
From what I've seen, Guild Wars has a far superior PvP aspect. The biggest reason is that because PvP takes place in special arenas, or in the Guild Hall for Guild vs Guild battles, there is an option to create a special PvP only character. These characters are maxed out with level, armor and weapons, and can use whatever skills your other characters have learned while playing the game. They can only go to the PvP areas, and take a few minutes to create one from scratch, making them great throwaway chars. Because of this, it's not a matter of who grinded out the uber lewtz to equip their characters, it's a matter of who knows the game and can adapt their skill choices and usage to what the other players are doing.
On the PvE side of things, Guild Wars is instanced while exploring, and then everyone hangs out together in towns and outposts. The instanced bit is nice, because you always know you can do whatever quest you need, because no one else could've completed it in your instance

And there isn't a silly 20 quest cap, which I always found absurd. You hit the max level cap about 2/3rds of the way through the first game, and about 1/3rd of the way through the second game. The good thing about this is that it allows the game designers to build a game that is balanced for the characters that will be there, and it allows players to concentrate on the story, and not have to worry about leveling up. The bad thing is that those who like leveling will quickly hit the cap.
Another little thing I love is that once you've been to a town or outpost, you can instantly warp there at any time by clicking on it on the map. Besides saving an enormous amount of time, this makes it really easy for people to help out other who may not be where they are. And it's free, you don't have to pay someone to fly you there like in WoW. With WoW, if you want to go somewhere, you either walk, take a boat or fly. Which ever way you take, unless you're going a very short distance, expect to send 10-30 minutes getting there. And you probably need to take a transfer or two, which is more gold to pay if you're flying.
Anyway, they are very different games. WoW seems to be more oriented towards those who like to play for very long stretches of time doing the same thing, Guild Wars is more for PvP or people playing for short bits.