The best I've used for carpets is ADA Aquasoil, but my only other experience in with Schultz Aquatic Plant Soil/Profile. Schultz worked fine, but is too lightweight.
Eco-Complete's different grain sizes and nutrients makes me think it is the best. Logically, the slightly larger grained top layer will allow the runners to creep while giving the effect of leaves hugging the substrate. I recall reading of some aquarists who felt medium-small grained substrate helped with low grown glosso. (Thoughts?)
IME the secret to nice glosso is more N, micros, light and CO2 than substrate, for whatever it's worth.