A lot can be said on the pros and cons between these two.
There's not much disadvantages with gravel.
For sand, one thing that can be a pain is that when they become compact, they create anaerobic spots. This alone produces much toxic gases thus killing the fish. You need to stir sand frequently to prevent compaction.
Brie is right about choosing substrate. If the gravel is sharp, then obviously your bottom dwelling fishes won't like it as their barbels get injured. Cories are the most vulnerable to this.
By the way, Brie,I have seen some freshwater stingrays in my two lfs but size is the problem. Even if I have to get another tank, this fish will still outgrow the tank.
They are very rare and a few people can afford them.