I have used AVG, but the UI always seemed to lack polish to me. I have no problem with how each one works, I just prefer Avast's interface (not that I need to use it that often). Avast also has a P2P and Instant Message virus checking service. That is something that I don't remember being in AVG.

I am not sure about AVG's licensing, but I'm pretty sure the license for Avast lasts about 15 months then you have to get another key. I have not had it that long to see what happens after that time.