The free version of Avast uses the same engines, (yes, engines) and database as the paid one. Other free ones usually partly cripple these aspects. Avast's approach is to reduce the number of features available with the free one, (firewall, etc) compared to the paid.
So, I'm using the free one, and feel pretty protected, having not had malware for at least three years.
I can't speak for the other free ones. Haven't tested them.
I very much doubt any of them "cripple" the engines or databases for their free versions otherwise users will think the brand is crap & not pony up for the paid version if they're so inclined. They follow the same route as Avast & offer fewer features. Avira certainly takes that route.