I've put scores of hours into both games and have come to think that Guild Wars is a superior game. The two are fairily different though, so there is a lot of area for subjectivity. Try both if you can.
Although Guild Wars is a beautiful game with a good amount of content, World of Warcraft is, IMO, a better game visually (I'd go so far as to say the best ever)--and, for better and worse, has a lot more content (e.g. you wont find trade professions in Guild Wars.)
That aside, I believe Guild Wars truly beats the socks off any other MMO simply because of the gameplay speed is much, much faster. Indeed, it's essentially an action-RPG except that in towns and guild halls many people can associate with one another. When you want to go to another town there's no screwing around, just open the map and click on a town and there you are. When you are trying to complete a mission there's very little screwing around, just follow the star on your map and the path before you (the game ultimately leads you in certain directions via mountains, gates, deep water, etc.. although you have a good amount of room to move laterally.) When you want to create a party of random people there is much, much less time wasted (that's what most people, in GWs, are in the towns for... and probably most people in your town are looking to complete the same quests.) There is no such thing as food, drink, and sitting for regeneration, you start regenerating fairily quickly if you've been out of combat for a few seconds. In only about 100 hours you can fully level your character; less with experience, help, and dedication. My point is this: the game is much, much faster than any other MMO I've played. This can't be understated. IMO, a game is fun--whether pong or a sports title or an MMO--primary because you're constantly having to make challenging decisions.
On top of that, the game is practically bugless (I haven't come across a bug yet, and the game is only a few months old!,) there is no monthly fee, and the PvP system is tactical, extremely well organized (from arenas to siege on other guild halls from your guild hall,) well balanced, and rewarding.