If I had the perfect answer, I'd be rich. Since I don't, and I'm not, take this with the proper grain of salt.
I tend to build for sponsors that offer recurring income, and that I think will be around for the long haul - your free sites will be around and creating sales long after you've forgotten about them, so plan accordingly.
If a sponsor shows impressive early results, of course I'll build more heavily for that sponsor for a while. OTOH, my current best performing sponsor as far as sales took months to kick in.
In short, it's the usual answer - what works for me won't necessarily work for you, etc.
