Having played GW for about a year (way back when) and playing WoW since TBC launch, it depends really.
Both of them are really easy to get into, as long as you know someone with the data-files. Otherwise, expect a nice BIG download.
If you like more realistic looking character models / area's and don't really mind playing alone online, I would say GW. This game is entirely free-to-play.
(based on the fact that I never really did "raiding" per-se and disliked GvG in GW)
If you like more fantasy looking character models / area's and want to play with other people online, I would say WoW. This game has a monthly sub-fee (which is worth it in my opinion).
(based on the fact that I have been in quite a few guilds with varying success in WoW)
There is a lot more to it than these 2 points, but as a starting point, I suggest those.
Other than that, people saying "play the trial - its free"... This doesn't really work that often as in 10 levels of WoW, you experience nearly nothing of the real game (not just level 80 stuff, but talent tree's, professions etc). I don't know what GW's trial system is like.
OH, nearly forgot, one more thing... A bit of a deterrent.. For both of the games, you could buy the vanilla version for a small amount (R100-R200), but you'd be missing all the magic they have added to both of these titles over the years. The expansions all-together with the game, could set you back about R400-00 or so... Just a heads-up.
My 2c