I feel sand is easier to maintain, it lays flat and its pretty easy to vacuum with a siphon. Gravel gets dirtier as waste gets between gaps and siphoning and sucking it up means you usually need to level it after as well.
Depends on the fish though, some fish I think like to nibble gravel for algae and food like Goldfish I think do it and some other kinds. Where as some other types of fish like corys, loaches and algae eaters like to do it with sand and dig around.