Anyone that tells you Aion is better than WoW is a dumbass, however, it is different, and doesn't suck, which by itself warrants checking out. Cuz WoW gets old after these many years even with new content patch coming, it's still the same thing.
Aion is fun and pretty, the PvP system is kinda like old wow pvp system, some people like it, but I wouldn't call it better. Character customization system is pretty crazy, which is also a plus. I just wish they didn't give the option to make tiny little fucking characters.