Wow is a better game for players with a lot of time on their hands. There is so much to do that you can literally play everyday and have something going on. Guild Wars is geared more for the casual player (which I like) as their content becomes rather threadbare.
Most of the differences between the two games comes down to personal preference. I've always been a fan of the Wow mythology and history and so that world is more appealing to me. I also feel that Wow offers more content, a bigger endgame experience, a ton of great dungeons to raid and remains a decent challenge. I also personally like the art, style and controls of Wow more than I do Guild wars. Also, Wow is just BIGGER. For the first timer, Wow seems so huge that you'd think it would take years to truly get to know it! And in some cases, that is true. I also feel that (most likely do to what you're paying) that the wow servers are run much smoother than Guild wars and problems are solved quickly. Updates and new content comes in the form of multiple free patches in Wow where as Guild Wars has you buy new games for big content.
But, Guild Wars is Free per month. It truly is the best bang for your buck- the game rocks and it doesn't cost a thing once you pick up your disks!!! It is the King of Free MMOs!
It really just boils down to what you like. 15 bucks a month is chump change in the grand scheme of things- if you love Wow and the entire experience it brings, then you have no problem forking over 50 cent a day to play. For millions, this fee is def worth the time they put in to it. The money also encourages Blizzard to put out new content on a regular basis and to constantly fine tune and expand their product. (which they do every few months) Guild Wars (at least to the hardcore gamers) is more of an MMO light. If you are someone who only feels like playing 1 day a week or so- you can do so worry free knowing that it doesn't cost you a thing to remain casual!
All about preference in the end.