I started with gravel and switched to sand which I much prefer. I have however got quite a few corys and they prefer the sand and it's less harmful to their barbels.
I have also switched from playsand (the cheapest) to black sand and my African dwarf frog is out much more as it is obviously better camouflage for him.
The black sand also shows off the colours in some of the fish better.
But again - it really makes no odds - it's what you prefer.
PS - not sure this should be in the emergency section mind!