I play both of those, but my favorite is Star Wars Galaxies. WoW is just too cartoony for my tastes, and really, the majority of the game is just glorified farming. It's kill x number of monsters, collect x number of stuff. Or find something by killing a random number of monsters until it drops. I think worst of all, you have to spend a lot of time walking from one place to another, and hunting for the elusive spawn of the monsters you're supposed to kill.
In Guild wars, there are a number of advantages over WoW. The music is much better, you can put a lot more detail into your not-so-cartoony character, and they don't skimp on scenery just to have no load time. In WoW, you'll notice that you almost never have to load, after the startup load. In Guild Wars, you have to load when entering and leaving cities. This allows for the much improved (and not so cartoony) graphics in Guild Wars. The missions are also somewhat more varied (not just glorified farming of dozens of different colorations of the same monster) and the team play is infinitely better. In guild wars, you have to have a party to do things, so the result is that it tends to be easy to find people who need parties.
Star Wars Galaxies improves on both of these in a few ways. It's like Guild Wars in that the graphics are good, but like WoW in that there is very little loading. Unlike both of them, there are spacecraft for the flyboys (or flygals) amongst us. The PvP is better as well. In WoW PvP consists mostly of ganking and dueling in mini-secnarios. In Guild Wars, PvP is dueling, and that's it. In Star Wars Galaxies, the Rebels and Imperials fight for control of cities, and entire planets. Otherwise its PvP is like WoW. Simply put, Star Wars Galaxies tends to have the best of both worlds, plus some. The only thing I dislike about it is that you can walk through most obstacles, though in some ways that can be a good thing. One time in WoW, I got stuck in a corner. It took me quite a while to find the auto-unstuck function in the help thing. STG avoids that problem entirely.
Edit: I believe you can play a trial version of Star Wars Galaxies and WoW, though I'm not sure about Guild Wars. I would recommend playing both of the trials before you decide.
Here's Star Wars Galaxies:
http://starwarsgalaxies.station.sony.com/index.vm?destination=trynow
Here's WoW:
http://teaser.wow-europe.com/index.html
You can download the Guild Wars client, but I didn't find anything about a trial access key, so I don't believe you'd be able to try it.