Avira has better signatures and adware detection but that's about it... As for Luke Filewalker, be better as Anakin Filewalker. Better name, better character, better Jedi. Even in the end of his life, he became one of the few (if not the only) to come back from the dark side. I'm a bit rusty with my Star Wars.
If I'm not mistaken, Avira has no web protection. Avast has very good web protection with many other tools handy to protect your computer. Performance wise, Avira is better. Recent versions of Avast have been harsh on HDD for some reason. Avast Free AV with Comodo Firewall is a very good combo. Avast also a bit heavier on CPU than Avira has been however on my computer, Avira is buggy. No one else seems to have that problem as far as I know so I wouldn't worry about that part.
My vote goes to Avast for being tried and proven over years of use.