Comodo is much stronger but you need to tweak it and learn how to use it.
Also, IMO you should couple it with a decent anti-virus (must be an AV without firewall and HIPS, to avoid conflicts).
So the truth is, you could do both: Avast free, plus Comodo free.
But to get maximum benefit from Comodo, tweak it like CruelSister says.
EDIT: she has a thread about it, read the thread (not just the first post) and you are set. Regarding Comodo
@MalwareTester
you wrote: "I was talking about Commodores AV not firewall."
The free version of comodo internet security comes with a cloud-based anti-virus that is so-so at best, and can be coupled with a traditional anti-virus (but double-check on that point. I personally don't have the AV component installed at all).
The free version includes the powerful firewall/sandbox/HIPS components.
The thing that makes Comodo great is not the AV component. It is the autosandbox feature that makes it a winner, plus a robust HIPS for those so inclined.
EDIT: so the bottom line is: if you are going for comodo for the AV, and you will turn off the other stuff, don't go that way. Avast is better in the AV department.