Personally I would say that Avast has improved and is much better than Alvira in the version 6..I have Malwarebytes, SuperAntiSpyware as free on demand scanners only....And I find that Avast version 6 free finds stuff that both SAS or Malwarebytes have missed ...I just started to notice this in version 6....Although Avast previous versions were just as good or even better on scans than Avira free....Also Avast shields monitor and protect you at all times and with so many shields, Sandbox, Network shield Behaviour etc. if you have it set to automatically do your actions like "move to chest" you should be as safe as you can possibly be...
Also just my personal opinion, but Avast offers so much more in the free version.....And the competition is trying to keep up with Avast and "Again" personally I think that Avira can't keep up and is forcing their users to upgrade....and looking for cash from advertisers...I also heard that their forum is full of unanswered monitored questions???
Just my opinion from the knowledge I have gathered that is being rumoured around...