Promix is cheap and more of a generic blend. Ocean forest is fairly expensive and a specialty blend.
OForest has basically everything you need already in the bag. Promix doesn't.
I use promix just because it's cheap. A whole compacted bale will go for $35. Mix it with some Earthworm castings, pearlite, bone meal, blood meal, and lime and you have a pretty good mix.
I dunno, the guy at the hydro store says he don't like the promix, but then again he sells the other stuff. Next time i'm there i'm gonna see how much it is and price it out. If it's a good price i'll probably buy whatever he has. If it's not i'll probably just get a bag of castings and mix it with the promix. (i get the bone/blood meal and pearlite at wallymart).