I have tried both and personally did not like AVG nor its past versions.
Avast I first used was 4.6 and it was ok.
In terms of usability both have improved, although Avast's old UI allowed skins which was a +, but now everyone is stuck to the same skin. Not to say it is not good, dont get me wrong, it is user friendly, but just doesnt allow customization of the UI unlike the old version which had the blue icon.
In terms of detection from my tests, Avast is slightly more superior.
In terms of Components, AVG Free lacks some features that Avast covers, but then Avast also has useless (in my opinion) features, whichs requires the user to do a custom install to get rid of them (unless you can disable inside Avast, but I never checked).
As for Usage, Avast is lighter.
Those are my opinions, so my vote: Avast.
Still willing to give AVG another try though since I am looking for some alternatives.