Here come the dozens of "GW is not a MMO" posts! Really?!
Since you said you expect to play on and off more, I'd say Guild Wars is definitely the way to go. That is one reason why I like GW, as I don't have much time to play it. In 2 weeks after summer semester and my job is over, I'll have a couple weeks of complete free time. Just like Christmas, that is a time when I tend to hit GW hard. So if I play for a few weeks every few months, there is no way I could play a MMORPG. I haven't played WoW, but seen others play it.
My biggest complaints with GW are:
1) Hard to find a party to quest with, as everyone is obsessed with standing in a circle doing dances and pointless crap, and trading. Plus there are so many quests it is hard to find people on the same page as you (unlike Diablo 2 where there weren't that many per Act).
2) Too many skills. Hard to understand what to use, and what is useless. It is not just that there is only 8 slots for active skills at a time... there is just too damn many!
3) Crappy items. I can't seem to consistently own in PvE, even if I'm much higher level than I'm supposed to be for the area. The items aren't as extensive as Diablo 2's uniques, rares, sets, etc.
GW is somewhat similar to Diablo 2, and ironically those three complaints I have are addressed in Diablo 2 for the most part. I can see where GW goes for skill-based, but damn I don't know WTF to use, and my characters generally suck, though my latest, a Warrior, is at least not completely crap for PvE (all that I play).
Otherwise, GW is a very solid and polished game, graphics are great, there is the original plus two standalone expansions, as well as another upcoming, plenty of players, lots of cool events throughout the year. PvP is suppose to be good too, but I've only played it a few times, as without a level 20... not sure I can do much.