adjustable lights should be used in place of under tank heaters because that is how the snakes are designed to take in heat- from the sun. Both can provide the proper amount of heat but there are more blood vessels in the top portions of the snakes body that will absorb and distribute the heat more rapidly and efficiently when a heat lamp is used. Not only is it physically better for the snake but it also allows the snake to recognize its circadian cycle and the light can be manipulated to create a proper breeding season by shortening the light cycle during the winter months then making them longer during the summer.
Plus it allows you to see in the enclosure which is always nice.