Well, I know sand is better for them, as they like rooting around looking for foods.
Barbel erosion, something commented about often, is usually from poor water conditions. I had all my Cories on Eco complete substrate for a long time, about 6-7 years. (some were in another tank with shrimp substrate, and another with sand.)
They perhaps didn't have the ideal conditions, this week I moved homes and changed their substrate to 3/4 sand.
They seem happy, they seemed happy before as well, regularly spawning. No barbel erosion. This sandy substrate will be better for them though and they can be even happier.
The Controsoil is smooth little balls right?
Is the gravel is it sort of smooth?