Blogs are good because the traffic is high quality, mostly from SEs and other blogs. Also, like others said, you can build a network and then get traffic going from blog to blog and of course getting SE traffic for each blog in your network.
On the other hand, free sites are also excellent. I mean, blogs have high quality traffic but in my experiences the traffic flow doesn't even come close to what is capable for free sites. If you get partner accounts and good free listings you can get high traffic volume. Yes, the traffic is lower quality... but there is just so much of it that it basically overrides the question of traffic quality.
Overall though, I am moving towards blogs more because right now I convert a lot better with blog traffic; free site traffic can be really bad at times. It also isn't as boring.
At the moment for me personally though... Free Site>Blog - although I am going to try to change that. (I just started with blogs in June and had free sites long before that.)
Maybe if I had as many blogs as free sites... my mind hurts now.
