Hmm, nice thread, as they are both examples of why everyone I know online (whom I all met in EQ, DAOC, or WoW) have fled the MMO market almost completely.
WoW did a lot of things right, for it's time, by stealing from everyone else and polishing the crap out of what it stole. People like to hate WoW, even those players who played it heavily for years, but the truth is that WoW had a lot of things going for it, including some form of early end game, that kept players in the game for a long time. The fact that the game basically continued to grow all throughout it's run until the cataclysm that was... Cataclysm, is a testament to what a juggernaut it really was. Did it revolutionize anything? Actually, I'd argue that zone phasing was semi-revolutionary, but otherwise, no. It just sorta took what others did, and did them cleaner.
WoW has a very bright, stylized (read as: cartoony) world with a very rich backstory and very well known characters, established via several preceding games and a few (actually not bad) books. There are things that I hated about it (Burning Crusade, Cataclysm, speeding up the pace of combat over EQ) and there are things I think it did really well (Blackwing Lair, Wrath of the Lich King). In the end though, it wasn't enough to keep the game interesting for 9 years. I'm not sure any game is, but, as much as I look down on WoW now, I have to say, I played it a lot, and something kept me coming back.
GW2 on the other: 3 weeks. Bored to tears. Couldn't care less. GW2 is everything that is wrong with gaming these days. If WoW sorta dumbed down the base skill required to be good at an MMO from EQ by introducing a lot of push-button skills with a lot of get out of jail cards and decentralizing the healing and crowd control skills (bad bad bad), then GW2 is the ultimate in that every single class is exactly the same thing with a little different skin. I tried to learn about the classes in GW2 and each one was the same "mash button, mash button, mash button... get loot." In the old days, when there was a REAL Holy Trinity in MMO's, it was tank, healer, crowd-control (really it was Warrior, Cleric, Enchanter). DPS was a secondary thing, because, of course, you always needed to do some damage. Now, there is no such thing as dedicated crowd control, and we're all worse gamers for it. Of course, in GW2, we don't even really need tanks, or healers, or crowd control, because everyone kinda sorta has that ability already. Thanks WoW for dumbing it down, and thanks GW2 for showing just how dumb we can get.
Also, bland blank nameless world with nameless characters in a nameless backstory that you can't actually make anyone care about. I tried to get into the story/lore of GW2 and found that it was so un-engaging and generic that I just stopped bothering.
Finally, GW2 is the last gasp of the tired leveling theme park that WoW really got started. Quest based experience gains in a linear level based system requiring you to progress through a quest storyline via zone to zone to zone? Sure, GW2 was a lot less linear than, say, The Old Republic (bad bad bad, same progression, same zones, same quests) but it was still the same "go to zone, do these quests... go to next zone, do variation of same quest." For the MMO genre to progress, someone's going to have to find something else (someone besides EVE who figured it out 10 years ago and is still growing).
So, to answer the question, hands down, WoW is the better game, for it's time (which has passed). GW2 is, umm... yeah, I don't have anything nice to say about it.