If they both work well for you then cost or familiarity and usability of the GUI usually become deciding factors. In a nutshell, whichever you prefer lol. Or flip a coin.
I believe F-Secure is better but IMO both are mid tier AV's there are better options that are still light. Then again though its your choice and as you've said they work for you.
If you like a simplistic UI & a product that just gets on with the job without you adding constant interference from its user - F-Secure - My fav AV/AM.