I was in your same shoes, i havent played WoW in almost 2 months. I am actually going to get both set up. TBH though, I am really liking Aion. It is close enough to WoW to fill that niche, but different enough to be something new. The main mechanical aspect that i am worried about is it economy, but i havent played enough to make an educated speculation on it.
It is hard to tell until i see its version of "Raiding"
I have 2 toon slots left if you want to take one for a day or two and try it out to see what you think.