I don't want to speak for everyone, but I do know that some of the WoW people that are playing Aion are rapidly losing interest in going back to WoW. However, there really aren't that many people playing Aion that were active in WoW - a lot of the people had either quit WoW or weren't playing that much anyway.

It's too early to tell if this is going to be another game like WoW that has real staying power. Leveling is SLOW, consequently we won't be seeing endgame for a few months I'd bet, and I think Aion's success will depend on whether or not they can keep people interested once they are done leveling.

I'd say, if you are having fun in WoW, play WoW. If you get bored in WoW, then by all means come give Aion a try.