Prepare for a very long / simplistic opinionated review of both games. (As well as some terrible grammar).
Most people would tell you World of Warcraft. To some people, this is still a good game.. but a mass amount of WoW players still play wow because it is "safe" and does certain things VERY well. However, the system is extremely old, it looks and plays aged, and at the end of the day, focuses intensely on end game content. Now most of the time, focusing on end game is great, however, considering there are now 90 levels, the first 89 levels will be incredibly, incredibly boring to grind through. There is no story that the game guides you through, you simply kill one thing, collect another, until you're 90. At end game, you have the choice to raid and pvp. Raiding can be fun, if you have a guild to run with. Most players now will say "There's LFR! You don't need a guild!" However, LFR - looking for raid, is more like forming up with 24 derps who have no idea what the mechanics are, only to power through the boss. In this sense, you are not experiencing the main factors that make a raid fun. Pvp comes in the form of random battlegrounds / rated bgs and arenas. Random bgs are okay, you have to grind for awhile before you can enjoy it, and there will always be days where your teammates are potatoes. Rated bgs are straight up boring, I really don't have anything to say for that. The "funnest" part of pvp probably lies in arenas. You find a team of 2 / 3 / 5 and fight other teams.
I have played WoW since Vanilla, and I have to say the game has changed for the good and bad. The feeling of adventure in WoW is none existent. 90% of the player base are incredibly rude. If you don't already have real life friends who play, I would honestly stay away from the game. With that said, if you have some established friends who are level 90, the game can be fun, they can guide you and show you the ropes, help you gear up and perhaps get you into a raid group / battleground group.
As for Guild Wars 2. I have tried plenty of MMOs since WoW's release. These include Lineage, Lineage 2, Aion, Tera, Archeage, Planetside 2 (maybe not relevant) - but none of them have captured me like GW2 has. I very recently hit max level, and am currently with the OCN guild on Fort Asperwood server. Not once have I felt as if I was just endless questing, because the way they implemented questing is unique. I won't go into details how, because there are plenty of websites that do this much better than me.. however I just have to say that leveling was a ton of fun. You have the choice of crafting to lvl, doing your personal story to level, along with exploring new areas to gain exp and map completion, as well as hearts (quests in a sense). End game dungeons have been pretty cool so far, of course, I'm still new to this, but the enemies are challenging, once in awhile there's some moments where the visuals just wow you. Of course, there are downsides to GW2 as well. This post is getting long.. so I'll be quick - personal story: your friendly AI towards the level 77+ events are fully stupid, and are not helpful when you get into a tough fight. Classes can be incredibly complex, the game can be overwhelming at end game, there are different "builds" your character can have, each build works better with one weapon than the next. To decide what sort of gear you want at level 80, you have to prepare yourself by knowing what kind of character you want, DPS, utility, control?
Wow, sorry for the extremely long post, but if you have more questions, just msg me - I'd hate to see someone spend the next 4 years playing an mmo and realizing at the end how much time they've invested only to regret it