Alright, I played GW since the week it came out, and was a HUGE fan of it, however, it's like a 3D copy of D2 (seriously, GW uses the same port as B.net, and Arena Net (the makers of GW) are ex-workers of Blizzard North), anyways, I had logged 360+ hours on my full GW account, over a period of months. For WoW, which I've had about 4 days, I've logged almost 2 days on it already, and still counting. Now, I beleive it's a bit too early for me to really judge on what is more fun, because I'm only currently level 16 in the game, but I enjoy every aspect of WoW, now this is where it begins to sound nerdy, so bare with me;
GW was fine, however, it was a bunch of kids who acted e-thugish, and were always cursing and calling everyone else gay.
WoW, kids through adults, most from what I've seen on TS with Sidoh, a lot of them are younger then me, so anyways, these same aged kids, spell almost perfectly everywhere I go in the game, no one curses like a blubbering vagina, nearly everyone's mature (nearly, we can't all have brains).
People talk like Pirates in WoW, so it's like a Madcox slugfest except without the flaming.
You can really feel like your character in WoW, you can match nearly every aspect of your true self to the game (this is where it gets addicting because it's like an upgrade of your real life).
I have not yet even seen Sidoh in game, however, I've played with Mythix and TSE an AWFUL lot, but me and TSE do a lot of clipping glitches and such so I don't expect this to get old fast (I'll post shit on it later).
But yeah, so far I've enjoyed WoW a lot more, and by the way, for all you mathhats out there, you can subtract the first month of payment from those numbers, because the first month is free, and if you order via bigger time frames, you can slim the price down by an awful lot, so quit your bitching, this game is extremely welcomed for anyone with free time on their hands.
I strongly suggest WoW over GW, there's just so much to do, and I can't fit it all into one day like I did GW, or even four (since that's how long I've had this).