Gravel can hold more crap if you dont vacuum regularily, and lower water quality.
On the other hand I feel sand is harder on the filtration since alot waste gets sucked straight into the filter which can lower water quality also.
It depends on the fish for me, diggers and very small fish get sand in there tank. Large messy fish get gravel. Theres just something satisfying about gravel vacuuming and seeing all the waste flow in to the bucket.