I've used ADA Aquasoil, Eco-Complete, Flourite, and Turface. All comes down to how much you want to spend. Eco-Complete and Flourite are about the same in price and in quality (both high quality inert substrates). Turface will be a lot cheaper, but it doesn't come in black, only in dark grey (which can be hard to find these days), and it also is very light in weight compared to the Eco and Flourite so it was annoying for me to plant in (hence why I don't use it anymore).
Right now I use Aquasoil in my 60-P and a mix of AS and Eco-Complete in my 120g. I guess my bottom line is, if money is no object, aqua soil all the way. If money is a concern, but you want something really nice, Eco-Complete.
Final notes: 3M used to make a product called Colorquartz that was like sand and came in pitch black. It's been discontinued as a product, but you can still find it in some pool supply stores. Also, MTS (Mineralized Top Soil) may be worth looking into if you are a DIYer as a way to provide a nutrient rich substrate.
