Avast preferable to Avira. However, for past few months I have used Microsoft Security Essentials which is free download from MS site. With MSE I find boot up is much quicker. MSE is a combined anti virus and maleware etc programme, ie makes Windows Defender redundant, and during installation will auto disable Win Defender.
The downside is that even with the highest detection rate of real viruses, no matter what the program is, your going to get a potentially higher number of false positives, files which your antivirus may accidentally mistake as real viruses. You decide whats more important to you, detecting more real viruses with slightly more false positives to the rest, or detecting less real viruses and less false positives? If youre not an avid downloader, then you shouldnt have much, if any, trouble with avira at all.
Its hard to work out whats best with the antivirus/firewall all in one suites, as so many websites seem to go with different ones as their all round favourite. If you read all the comparison websites and choose the antivirus/protection program with the best ratings, one may still let you down in an area of protection that others will exceed in. its getting confusing with so many spyware programs also trying to become fully fledged antivirus programs, but if you go for one of the most common top 5 all round protection suites you should be fairly secure. I just use free software (!) and use Avira as an antivirus, with malwarebytes antimalware as a backup antivirus-also with superantispyware or adaware installed to detect spyware, and finally spywareblaster to prevent bad websites and activex/related internet attacks.