I've been using Avast for about 5 years now. It's excellent. It has actually caught something Norton missed. Avast will however detect false alarms, but personally I would rather have that than skip any. I have installed Avira on a couple systems - so far so good, and I like how you don't have to reset the key every 14 months. As for performance, it's too early to tell.
Put it this way, you already have Avast, it's highly recommended, (especially on this forum). I've installed Norton on a few systems by force... just keep Avast.
That is my personal opinion.