The answer is going to be subjective. I've heard that f-secure is a good program, and reviews are usually good.
At the Avast forum you are mainly going to get reports from users who are very happy with Avast, including me.
Since both (should) have a trial period, why not try them both out and settle on the one you like to use the best?
Considerations should be how easy it is for you to use, what the system load (or slow down) on your computer is, and how good the support is.
I don't know how f-secure's support is - pretty good I would suspect - but Avast's forum, often visited by Avast developers/gurus, has to be one of the best.
Personally I wouldn't be surprised if, overall, Avast is one of the best AV's, anywhere, period.