Avast has webshield, autosandbox, behavior shield, website reputation addon, script shield and network shield. Avira doesn’t. Also avast! has ability to repair some types of malware and you can set-up the list of blocked websites. Also Avast! can compete paid antivirus suites. Avira can’t.
But Avira has better detection rate than avast and is has much much much faster scanning speed than avast!