If you are using a hydronic heating system, say heated by gas, the closer the tubing is to the surface the faster the heat will get into the room. If the heat source is solar collectors, then you might want the tubing deeper so that the sand is storing the heat for evening. This leaves the radiant tubing easily accessible in the event of any leaks. But invert the whole mess. The problem is that the sand doesn't conduct heat well.
Heat transfer depends directly on the heat conductance and on the temperature difference.
If sand conducts heat half as well as concrete, then for the same thickness of sand, you have to run the circulating water at twice the temperature difference. If with a concrete floor you have to run 90 degree F water to maintain a 70 degree room, then the temp. If the complaint about concrete is the high embedded energy to make it, use a thinner layer. For foot traffic it should be possilbe to use a well consolidated road pack base, and put only 1.5 inches of concrete on it. Another possibility is to use sand, but to eliminate the air spaces in it.