Flourite makes a sand substrate other than the Black Onyx. http://www.seachem.com/Products/prod...BlackSand.html
My roommate uses it and his plants seem to be doing alright.
I use flourite black (not the sand) in my 20H and floramax in my 20H. The size of the flourite is much smaller than the floramax and seems to weigh a lot less than the floramax. As far as which is better, I can't really tell. I would assume flourite is better from what I hear from other members. I usually hear of people using flourite, eco complete, or aquasoil when people mention their tanks, but never really floramax.