ProMix is an inert medium where as FFOF has all types of organics in it already. For someone new to growing I would reccomend FF because all you do is add PHd water for say a 4 -5 week veg cycle. ProMix has to have nutes added to it almost from the beginning to keep the plants going. Both are great mediums. Nothing wrong with MG soil either, I have grown many a good plant in that stuff, even the soil w/ slow release capsules has grown me some dankness, but I DONT reccommend slow release soils because the plants have to be heavily monitered and almost always run a 90 day finishing time with only 20% amber trichs. I suspect that it's the low ph the soil retains all the way up till the end.
... Snuff