I use belly heat. The ambient air temps in the tubs can have small differences with just back heat. The belly heat provides more of a temperature range. For example the room temp is 75 the tub ambient air temp is 75 cool side 80 warm side with a belly heat of 90. If you use back heat the snake will not be able to access a larger thermal gradient. Also, for cooler rooms I prefer belly heat.

I guess this depends on what you are looking for. Temperature stability or larger thermal gradient. I prefer the latter for my applications.