Free an paid antivirus editions are the same.. Free editions download the same update deffinitions as their paid counterparts. They have obvious differences like limited Configurations and don't include a firewall. You can configure a paid antivirus to update at the hour or day you want, the free editions can only update right after logon, you may be able to delay it but not set the hour. This can make the boot process longer and have the OS ready for use later in the logon sequence particularly if the computer is low on resources.
I use Avast free and have used it since the Avast 3 edition... never had a serious virus infection, the only serious event was with AVG in 2006 and this and some mild infections were due to downloads, but that possibility is now lower since Avast scans every download and pauses/stops those it considers suspicious.