Malwarebytes is an anti-malware application rather than a full blown antivirus, as you say, I think John was comparing the quality of their support to Avast rather than the actual application
The major advantage I find with Avast is that the Free version is the same as the professional version except for certain additional functions that are better suited to a commercial managed environment - Scheduling of scans and reporting etc.
As for X64 compatibility of the "Free" version 4.8. I am running here on RC without any issues at all. The only issue I've ever
had with Avast in the years I've used it is the volume of the Update message