I think Guild Wars is actually free to get online (mind you, this conversation from which I heard this was held over a year ago, before the game even came out and the Beta was released)
The comparison was between WoW and GW, but I'll try to see if I get this right (don't kill me if I don't.
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WoW - Have to pay for it in stores, and have to pay the monthly fee (unless you buy the packages in which it lasts longer than that), is more dialup friendly (this is the part I'm not sure on). Is less picky on what your computer has to have to play [lower System Specs required].
Guildwars - Costs to buy it in the stores, free to get online and play, but downloads patches constantly (from what I was told) and could greatly bog down any dialup user. Has higher System Spec expectations.
Now, this is from before it was even released, so a lot could have changed or I could have mixed something up between the games.
Talking to friends that are hardcore MMORPG players (I'm not really, but I had to listen to them talk about the games EVERY. SINGLE. DAY in PE class. >_<), they got tired of both after a short amount of time, but most of them playerd WoW a lot longer than Guild Wars. Though, personally, since I'm cheap, I'd probably get Guild Wars since it doesn't cost to play.
(except to buy)