I couldn't say, I've played Aion for only a few hours and it just feels better, I guess it's just the environment, I love playing in beautiful areas and Rift was seriously lacking in this area, during the first 25 levels anyway.
Probably not enough to make you like Aion if you don't like Rift though !
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Best LPer ever, Gix ! : http://www.youtube.com/watch?v=Vt5YKmP7ecY