I believe the choice of substrate is largely dependant on what you are keeping in the tank (plants, cichlids, messy fish etc...), what type of substrate your using (gravel, sand, size etc...) and what type of filter (Canister,
UGF...)
In my case, I have a 55gallon planted community tank with a
HOB filter. I also have loaches and cory cats who like to root around in the gravel. I chose a medium/fine natural gravel (from jade beach) with 2-4 mm stones that are smooth. With about 50 pounds in there I have an average of 1.5" to 2" deep gravel. I find this to be a little thin for planted, and actually wish I have put about another 15 pounds in there.
Before I had plants, I had a
UGF in there, for that I used 25 pounds of course aquarium rock, the smooth coated stuff. This seemed about right for that. In another tank with no plants and a
HOB filter, I have about a half inch of the course rock. I like the larger rock in there because I can get aggressive with the gravel vac and not suck the rock up so much. The smaller gravel in my main tank, you have to be real carefuls not to suck it up the vac.