I have tried both. It is very intimidating the first time you look at WoW of a high level player, it looks like there is SO much on the screen. the reality is though, I only use some core skills most often, and all the rest are for intermittant/very infrequent use.
I hate the idea of pay-per-month, so I really tried to give GW a genuine chance. I stuck to it for over a month. I found that the quests felt extremely repetitive, with only 3 or 4 different types of quest scenarios most of the time. Then, when some guild mates tried to help me providing better gear etc, I found it very difficult to figure out what gear was better than others (the descriptions help little in determining which is "better"), especially since the differences between low level and high level gear is so slight. I also found I had to ask a million questions about everything, the game seemed rather difficult to learn even the basics. there was absolutely no "tutorial" basic early-game to bring you up to speed, where I felt WOW did a much better job of that (although could still do better).
In fairness, I have been playing blizzard games for years, so I may be too used to their style of gameplay. However, I do not really agree with others opinion of the graphics. I feel that most of GW is in really wide-open areas with a camera WAY overhead, very slow and repetitive combat, and little to look at. With WoW, I find myself taking screenshots when I find nice new vistas, waterfalls, etc.
pay-per-month is really lame. I hate it. I also hate the crazy travel time between some areas of the WOW world. It is not my favourite game I've ever played (in fact, I think I preferred WC3 and Diablo2), but I do enjoy running instances with friends and playing together. I think these games are going too far towards PVP for my taste, where I prefer teamwork and co-op play.