World of Warcraft.
Both have a free trial so you can play both and decide for yourself, although they are capped, so don't expect to experience everything. I played Rift for a while, I realise the end-game is lacking due to it being a new game, but it's going in a direction I don't like. Everything is far too structured for PvE, the zones are quite small and force you to level along a certain route, the dungeons and raids lack difficulty, the professions are insanely easy to level and the huge talent tree customisation (which they love to brag about) comes down to making a macro that uses every ability in a set rotation. A lot of the skills that you learn aren't even worth using because you don't spec into the improved version. The Rift's feel like a gimmick also, the invasions are small scale, it's kinda cool to team up with the enemy and take down the boss leading the invasion, but the impact of the invasion and Rift's themselves feels quite small. I stopped doing them at 50, they just got tiresome. It's not "oh cool, a Rift" it's "for fuck sake, another?"
I'll admit that the dungeons in Rift are amazingly designed, they look fucking beautiful, the same can be said about the entire world too. Graphically it looks awesome, and I like how they're currently adding options to customise your character's appearance. The weapon design and armor design for me are unimpressive though, they went for a realistic look which, in a fantasy world, looks out of place. I wouldn't recommend Rift either if you want to PvP, there is little world PvP other than ganking people outside the lv50 quest hub and the Warfront's are a clusterfuck of imbalance, it was a little bit of retarded fun for a bit though, and the ability to speak to the other faction allows for some funny smack talk during combat.
WoW is just better in my opinion. More content, more freedom, appeals to both casual and hardcore players, better PvP, better lore, better art direction, better dungeons/raids. I know it's down to opinion but Rift just feels lacking, a lot of people I know who have played it have said the same thing. I don't mean lacking in the sense of content either, it's just missing something. I quit last year and renewed last month to see what had changed, it still feels the same.