I would stay away from the sand, just because it tends to get everywhere and even in their eyes. Some people do use it successfully but i really think it is just to messy. Cypress mulch has worked best for me.
Just sand would be too big a risk of impaction. If you have it mixed in with the dirt or coir then it lessens that chance to basically nothing especially with a well hydrated animal. Helps too if you feed them on something rather then on the substrate. I myself use the coir, sand, then have some cypress areas and leaf piles (well until the tortoises come along and mix everything together. ).