As a defector from AVG over a year ago, I would say no contest, avast! is the winner with a much more flexible/configurable, multi level protection (Web Shield, Standard Shield), multi item protection (standard shield, web shield, net shield, email, p2p, im, etc.).
Yes that flexibility/configuration requires a little investigation/research on the part of the user, but it is very worth while. The research (read the help file, sticky threads in the main forums, etc) will further enhance the experience/protection of the default installation settings, which for the average user should be fine.