For me there is one major deciding factor between WOW and GW. This factor is the $15 fee.
I dont care how good a game is, there is no reason why I should pay money for a game after having already paid $40-$50 for the game itself. Some people are going to say that the monthly fees are to cover the server maintanance costs and to root out hackers. Well, Blizzard and many other companies had no trouble maintaining their servers in the past without milking us for money. Diablo 2 was freee to play online, and so is GW. I'm sure Anet(company behing GW) spends money to maintain their server and to root out hackers, yet they are decent enough to understand that money dont fall from the sky for most gamers. But I'll stop ranting, since I'm getting carried away here with my anger. *Takes deep breath*
Ok... The bottom line is, Guild Wars is designed to be a mostly PvP game - NOT a roleplaying game (even though it has a RPG portion). Unless you like PvP, and you want to PvP all the time, you have no business playing Guild Wars. I'm not saying it to be mean. It's the truth. GW beats WoW when it comes to the PvP, since the GW system is deeper, and favors the player's skill, over "godly weapons" or "l33t items". In GW, it's all about your skill, and no "Uber Fiery Demon sword or Annihilation" wil help you win in PvP. That's the best thing about GW.
WoW on the other hand, is more about the RPG, and grinding for items. In that respect, WoW wins over GW hands down.
So please concider what I said, and this should be enough for you to decide. Hope this helps.