I've played both.
WoW will give you a more typical MMO experience, and will allow you to play around with races/jobs with more depth.
Guild Wars will give you a much less typical MMO experience (think Diablo 2 meets WoW), and will allow you to have a more 'fast-&-fun' experience.
Guild Wars will stop you at lvl 20... it's the highest level you can (currently achieve) but... as odd as this may sound... lvl 20 is actually where the game REALLY begins.
WoW is a race for the highest level, & best gear. Guild Wars is all about the skills you use, how you use them, and teamwork. Sure, there are weapons that are better than others, but there is no single 'uber' item in the game. Everyone is on a relatively even playing-field, from lvl 1 all the way to lvl 20. What changes, from group to group, is how good a player you are, and how good your allies are.
I have been in teams where the lvl 7 Monk/Mesmer is a MUCH better ally than the lvl 15 Warrior/Ranger... because the lvl 7 is using skills that compliment themselves better, that aid the team more, and is employing good strategies/tactics in each battle... while the lvl 15 Warrior/Ranger is using crap skills, and running into each battle blindly... often resulting in them requiring a ressurect from the lvl 7!
![Smiley :)](https://forums.questionablecontent.net/Smileys/qc/smiley.gif)
And that's the other beauty part of Guild Wars; as a lvl 10, it's not uncommon to be in a group full of level 20s... and still be very useful. The levels don't matter too much in this game.
Even though WoW will run better on most computers, I still stand by Guild Wars as being the game to play. Zero monthly cost, while still having the MMO experience... plenty of crafting/customization options... and one of the best stories ever in an MMO.
You really become invested in the characters around you, and in the story... you actually start to give a shit about what happens to these NPCs... and you find yourself enjoying helping all these people in side-quests.
With WoW, I found I would enjoy myself for about 5 minutes until some random player ran by me yelling "OMGWTF!!! I AM T3H UB4R!!!!!11one! LOLZ!!!"
Guild wars retains a constant and surreal sense of beauty, and environment.
WoW suffers the fate of all Blizzard online games; a community predominately populated by asshats.