I play both GW and WoW.
GW has it's own perks. We've already mentioned one being it's free. 2 It's absolutely beautiful. for a game to have a good replay value and sweet visuals, to me is a catch. The first time I played GW I was expecting something more. At first GW is fun. As you get to about level 10-15 with your character is starts to slow down quite a bit.
Then WoW is a given. First of all it's made by a reliable company that we all know and trust. I'll be honest. I have only been playing the 2 games for a week so i'm just going on first impressions. I really like them both to be honest. WoW is more addictive though and has quite a few more things to do.