AVG vs AVG Free
There are no significant differences. There are a few things that load and scan automatically in the paid version, that the free one does not have.
Unfortunately, the free one will probably no longer be available after some time in January. They got big write-ups in the Washington Post, Chicago Tribune, and New York Times. Now they have 30 million free users in US and Europe so they cannot afford it, and they don't need to do it free now.
It remains excellent software. The cost is $35 for two years, and it beats the pants off Symantec, McAfee, Trend Micro, in my opinion.
So unless they change their mind, the big difference is that Free won't be free anymore. Avast is the last of the free ones.