A couple things. Heat lights & heat lamps are different. One emits heat & light, the other (heat lamp) emits heat only. Out of the two, I would say heat lamps are generally better. However, this leads me to my next (& main) point. Heat is heat. As long as the snakes body temp is warm enough to digest, it doesn't matter if it's under the tank, on the back, over head, or ambient.
Some people will say that it *has* to be belly heat,but I disagree. I know well known, successful breeders that also use ambient heat. I know some that use back heat (heat tape along the back of the racks). I myself use ambient heat.