I have heard a couple of things:

Belly heat is better for a rack that will be set up in a room with temps that can drop below 75 degrees.

Back heat works just as well or better for racks housing smaller tubs, as it provides a better horizontal gradient. The temps will distribute evenly from the back to the front of the tub, as opposed to belly heat, as it will provide more of a concentrated basking spot in a more concentrated area.

Back heat should not be used for deeper tubs, as it will not provide enough heat to reach the cool end.