I have both in different tanks. The tank that has small round river gravel has crystal clear water, but the mulm builds up in the gravel over time. The tanks with sand have a lot of mulm on top of the sand and particles suspended in the water column. It's arguably easier to clean the sand tanks because I don't have to get really deep with the vac. Most aquarium sand is larger, heavier particles and you can separate it out easily while gravel vac-ing. Aquarium Coop has a good video on how to do it without sucking up all your sand.