I have now tried MiracleGro organic, Fafard regular and organic, Espoma organic, and Al's (tapla) 5:1:1 bark:perlite:peat mixes for wintersowing.
I didn't mean to start an experiment, but these are just the bags that were either left over in the shed or were on sale.
Espoma is my least favorite and I will not be buying it again. It turned to hard brick as soon as it got a little bit wet. I don't know how anything could grow in it. Fafard is my favorite. It has this beautiful silky texture when wet. Al's mix was pretty good, but I hadn't done good job of sifting out the bigger bark pieces. If I put the energy into sifting it a little better, I think it has the potential of being a really good mix, and cheaper than pre-made.
I had just made Al's "gritty mix" (Turface, grit, and bark) for my succulents, so I had very small pieces of Turface that I'd sifted out. Instead of sand, I sprinkled Turface fines over the seeds. I'm hoping the Turface helps with moisture retention this spring when things start heating up.
I still have a bag of ProMix that I'll be using for spring sowing. I wonder if I'll like it as much as the Fafard. Now I need to see what's more cost effective!