I don't think avast detection is average, in total (signs, evo gen, deepscreen etc), why do you say that? If you mean only about signatures, you cannot say that only these make a product effective and i believe you know that. xD
Actually i would like to mention that the comparison between Avast and F-Secure is like comparing apples and eggs. In other words its BS.
However if we have to nitpick then i agree a 100% with daZa9 he is correct by saying that F-Secure has a edge over Avast due to the whole package combined.
Perhaps that individual modules within both packages could be subject to better or lesser performance compared to its counterparts but F-Secure really does have unmistakable edge. But then again the whole focus of F-Secure is different then Avast so F-secure is in a league of its own here and Avast really does not stand up against it.
But this has nothing to do with the quality of both programs, but more the individual focus and protection strategy behind the program itself.
That said there is however ONE field that F-Secure has a serious advantage and that is its Zero day protection, it is so much more robust and well rounded then Avast that i am not even going to take the trouble to explain the differences.
i hope this helps.