Minor add to last:
Those results at av comparatives (respected testing site) reflect the performance of the "paid for" or "pro" versions of AV's.
So in that respect, Avast probably scores better, because the detection/cleaning rate of the free version is identical to the paid version.
With AVG and Avira, the rates between free/paid differ. Don't know how much, because I haven't found a reliable site that comprehensively tests freeware.