Generally, the darker the substrate, the better the fish and plants will stand out. Black looks best imho. Eco-complete, onyx sand, and natural soil are all black substrates that look pretty nice and are beneficial to the plants.
Are you plannin on mixing something with eco-complete? or are you going tokeep the substrates separate?
amano sometimes advocates the use of white sand in the foreground. have a look
. It may be what you're looking for. I've never done anything like this but amano advises putting a rock or driftwood to act as a divider for the substrates. you dont have to do that but it would make things easier with doing water changes and help to keep the two substrates separate.
i have this stuff called volcanit
that looks pretty good. I havent mixed it with the eco-complete but i imagine that it would look good. Volcanit ranges from dark greay almost black to a deep red. its good for planted tanks, its porous, doesnt affect pH and its little finer than ecocomplete (which is good for some plants, growing gloss on ecocomplete can be a royal pain!). Its about the same price as the eco-complete though, so if you're not looking to accent the black with another color you might jsut want some more eco-complete.
From experience, i can say that you can use flourite as a base and spread the eco-complete on top. the two wont mix and will stay seprate (until you disturb it yourself). I did this on my 20H and it worked great for over a year. My thinking at the time was that the flourite was beneficial to the plants and its a bit cheaper than the eco-complete stuff. However, this past weekend, i had some plants delivered and i drained the tank to re-plant it (and thin out my tenellus jeez that stuff spreads like a damn weed!). To make a long story short the two layers are now one, and it looks pretty bad. Its not horrible, but its definitly not desireable. oh well.
It all really depends on how you use the different substrates. some look good mixed. some you can layer (then it doenst reammy matter how it looks) and some you can keep seprate for a contrasting effect. good luck! Hope this helps you.