i have been experimenting... which to me is the best way to learn anything. I have 4 tanks at home, and 2 at work. here is a breakdown, and a little summary of what I've learned:
50 gal. African Cichlid tank: crushed coral mixed in with gravel- I didnt put this in, the previous owner did. It looks to me like it is crushed coral, either way, it raises the PH which is the vital to having African Cichlids.
30 gal: black pea gravel- perfect for cichlids. black gravel brings out color better. This is the regular run of the mill substrate.
50 gal. breeder with GT, and firemouth: natural looking pea gravel. once again, the run of the mill regular substrate you get from the pet store. good for cichlids in my opinion.
125 gal. community: large sized pea stone mixed in with flourite. This is a clay-like substrate that is perfect for the planted aquarium. it is chalk full of minerals that are good for the plants. it looks pretty good too. my plants of flourished.
30 gal. exodon tank: eco-complete substrate. this stuff is gold. it comes in black water with beneficial bacteria in it. it helps cycle your tank. it looks like black soil, and is beautiful. it doesnt cloud the water whatsoever and does not need to be pre-rinsed. it is the perfect substrate in my opinion, and I wish I had the time/money/motivation to change all of my tanks to it.