When I set up my 75g, I used 80lbs of Eco, since that was all my LFS had in stock, and 60lbs of Tahitian Moon Sand. The sand is nice because it really helps hold some of the smaller foreground plants (Dwarf Hairgrass, Hemianthus callitrichoides, etc) in place a bit better then the Eco alone. Some say Eco is too light. In most cases I would disagree but with small foreground plants, the Eco just isn't enough to hold them down. If I were to do it again, I would add another 20lbs of the sand.
The black sand also gives the tank a darker coloring and smoother texture than the Eco alone, which I like. I guess that is a matter of opinion. The sand will eventually get mixed in with the Eco, but I figure when it does, I will just add a bit more sand. As another plus, the sand is about $10 cheaper for 20lbs in my area.
If you decide on flourite instead of Eco, I have had good luck mixing Flourite with Schultz Aquatic Plant Soil. They are similarly priced but the Shultz has more iron and a much greater CEC. It is very light though, and the Flourite will add some weight for anchoring the plants. The Schultz is also much, much easier to rinse than the Flourite. Try doing a search for Shultz or Profile (same thing) on here. The Schultz/Flourite option may save you some money over the Eco.
For a depth comparison, I used 2 bags of Schultz ($14 @ walmart), and 1 bag of Flourite (~$20), topped with 1 bag of Flora Base in a 55g and this gave me a 3 1/2 - 4" substrate in a 55g. You could leave out the Flora base and just add another bag of Shultz or Flourite.
Shultz and Flourite are very similar in appearance (when wet) and size. Once they are mixed together it is hard to tell which is which. If you use the calculator gnatster gave you the link for, just use Flourite and then you can figure out how much of each you want to add depending on your budget and desire to rinse.
