They would probably like sand better, because thats what their natural habitat is made of. Poo will stay on top of the sand, which looks bad, but is sooooo much easier to remove by just siphoning it off with water changes. Or, you could go all the way and set up some under gravel jets to get a flow in the direction of your filter intake (will work with gravel also btw)
After asking that very same question here about a year ago..........I would never go back to gravel again. I got my sand froma local mountain lake and have never been happier. The poo in my tank doesn't really collect anywhere as there is alot of flow.