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Seleya
Wall of text incoming...
So I've played both, but stuck with WoW because of personal taste. I've always felt GW2 seems to be more focused on PVP endgame (I could be wrong), but both have their strengths and weaknesses, so I'll just try to focus on your interests:
Character Development/Roleplaying - GW2 has a very strong intro questing sequence and amazing character customization for every race. It also uses a story customization piece when creating a character, which can be good or bad depending on how you look at it (essentially it provides you with a choice on a piece of your backstory/beliefs). Most of WoW's intro questing sequences are not as strong, but there are a few (Worgen, Pandaren, Goblin, and the Death Knight intro) that I feel are almost as strong as the GW quests.
Later in the game, it pretty much comes down to this distinction: WoW focuses much more on you being a small part of a larger story (so fostering story through cooperation), and GW focuses on your personal story (fostering story through independence).
Another thing to note is WoW has quite a few realms which are specifically for RP.
Exploring/Questing: WoW's world is huge. And when you get into the higher levels, the art starts to really shine. The questing in the game is pretty standard, some guy tells you to do something, you do it, and he might give you another quest. But this system leads you through an individual story within each zone.
GW2's world is fairly large and the art is pretty varied. The questing system is a bit odd, you complete favors for npcs by doing specific actions within their territory. This gives you less descriptive stories and a feeling of openness. This mechanic (along with a Points of Interest' mechanic) is meant to foster open exploration through the worlds.
Combat: GW has a much more fluid fighting style, you have to actively dodge attacks and there's no true 'healers' in groups and focuses on positioning and enemy movement control. It uses a unique weapon system that allows you to switch between two main toolsets on the fly.
WoW's basic mechanics are much more simple, just fire off abilities without too much worry about positioning. Where WoW does start getting interesting is the 'specializations', where every class has three or four main toolsets meant for doing different things. Each of the toolsets are distinct, a Rogue might choose to be a stealth or poison specialist, or forgo that for a 'swashbuckler' style set. WoW uses the common rpg trinity of tank/healer/dps.
Art: I'll say this now, WoW will age better, hands down. It just had a graphics overhaul for its characters. The aesthetic is pretty standard, except for Pandaria.
GW2 has a varied aesthetic, the Charr are dieselpunk, Humans are pretty standard, and so on. So if you want something different, GW is probably better.
As a final note. GW2 is cheaper. You pay once for the main game, and no more. WoW has a monthly subscription model. You can try out the first 20 levels of WoW for free though.