Sand is easier to clean, the muck and mulm stays on top. With gravel, it all seeps between the rocks and is a horror to clean, especially if you disturb the gravel.
Sand turns black in spots when plants or dead animals are trapped in it. But mixing it and siphoning the water will clean that. My plants love sand far better than gravel, much more natural too.
Sand is better for all bottom dwellers, more smooth and safer for the ones without scales.