It's really a personal preference.
I find sand easier to clean. With sand, you just use the tub part of the syphon and hover above the sand. All of the debris get sucked up easily. With gravel on the other hand, you have to use that big cylinder and push it all the way in the gravel and then do that for the whole tank.
Also with sand, all the debris stay on top. With gravel, all the debris get sifted down between the rocks, making it harder to get all of it.
But again it's just a personal preference. Some people like the looks of gravel, while others like sand.