Sand or gravel or both?

[BeacauseImAnElf]

We have just got our new tank. It came with sand and we have gravel in our tank at the moment.
I'm a bit unsure of using sand and gravel. The man we got the tank from said to put gravel on top but surely when we clean it the sand will come through?
Our clown loach is a little bugger and digs everything up as it is and I don't want the sand to cause him any problems.

Don't mix them it'll be murder to clean!
I prefer gravel as its a hell of a lot easier to clean with a gravel cleaner, but it's more whatever suits the fish you've got if you've got any fish that just skim along the bottom like corydoras then go with sand as its better for their tummys but clown loaches are fine with gravel.