I've been a long time subscriber of Fred Langa's Windows Secrets newsletter. Recently I highlighted his article on Crapware. I also just described my experiences with Microsoft Security Essentials v4.
In an article in the current edition of the Windows Secrets newsletter, Fred applied the process he used in the Crapware analysis to the resources required of the various free anti-virus solutions. Fred comprehensively analyzed 6 free anti-virus tools for their impact on startup time, shutdown time, RAM footprint, and disk space.
There was no runaway leader. Microsoft Security Essentials didn't win nor lose any of the measures but was always respectable. Clamwin had more wins than anyone else.
I'm not sure there's enough difference among the leaders to make anyone switch from one to the other. To me the most interesting part was Fred's process using Oracle's VirtualBox.