I would recommend ADA Aquasoil - Amazonia, hands down! It is kind of pricey, but you can order a 3L (10lbs) bag for $17.95 (w/ Free Shipping) from ADGshop. 3L was enough for me to fill my 2g Cube (7.5x7.5x7.5") sloping the substrate from its heighest point (3") to its lowest point (1.5") and I still have 1/2 the bag left over.
I also have it in my 50g mixed with Fluval Plant Stratum.
From my experience, it's small, doesn't breakdown easy, and has yeiled the most growth from my plants in the the fastest time, as compared to the Fluval substrate.
Just my 2 cents...