I have Avast on some of the seven family computers I support, and AVG on the rest of them.
Mozilla Firefox with AdBlock Plus is on all of them.
The above programs are free and everything works well.
As I recall I switched my wife's laptop from Avast to AVG (maybe it was the other way) because she was complaining of slow boot up. We take that laptop out to the ranch then home again so sometimes it hasn't been online for a couple of weeks. When the virus scanner was way behind in its updates, it took a long time to download the update, install, and do some kind of preliminary scan, before the machine could be used. I never notice this on my own laptop which is running Avast, but this may be because it is used more frequently.
Aside from this slow bootup issue, I don't see any difference. I think both AVG and Avast are in the top group of free virus scanners.
I suspect the AdBlock Plus deters a lot of mischief because it intercepts requests to download popups etc, that otherwise would arrive along with what you intended to see.