I beg to differ, but AV-comparatives.org has done a recent test on Avast and Comodo.
On Avast, it last tested in August of last year and it missed 2% of all then-current malware, versus 0.2% for G.DATA and 0.6% for Avira (the free one doesn’t miss any important stuff from their retail one).
http://www.av-comparatives.org/index.php?option=com_content&view=article&id=144&Itemid=152
A speaker from trend reported 100,000 new malware signatures were released every day. I’d imagine those comparisons depend a lot on when they were tested.