Also, Know that they are both not very good removing nasty viruses from the system. For example, I tested sirefef genC on virtual machine; Avast could detect the driver infected but could not remove it, Avira could not even detect the virus. I know it's a particular case, but it's one of the most known rootkits these days and it's possibly an idea how products handle such difficult infections. I also tested Kaspersky, Norton, Webroot. They detected the rootkit but could not remove it completely either. The only program that removed it properly was microsoft security essentials.
Avira seems to be better in AV-comparatives removal tests than Avast but take it with a grin of salt as a big testing company is involved.