Frankly I have not actually read or studied anything that says DEFINITIVELY one way or the other, but this is my thinking for why belly heat is better for their digestion, BPs are nocturnal, meaning they dont get radiant heat from the sun(aka radiant heat from back heat), so how do they get heat you ask? from finding things that were heated by the sun during the day time and still hold that heat into the night time..say a rock or exposed patch of sand for example..so they go and lay on this rock that was heated during the day time and where is it heating them from? The belly..so thats why I think belly heat is better for digestion..it just seems like thats how it would happen in nature. now I'm not in any way saying this is EXACTLY what happens EVERYtime because I'm sure that it often doesnt, this is just my own personal rationale for using belly heat.
Also on this subject I think the size of the tub should influence the choice between belly and back heat..mainly because if you're using a shoebox rack, a strip of 3-4" heat tape would cover too much of the tub and could essentially make the entire box too hot and wouldnt provide a good thermal gradient for the snake to thermoregulate, so in the case of a shoebox size tub I would, in fact, recommend back heat. However on the case of a large tub, say a 41qt sterilite, I would always recommend belly heat for the simple reason that a 1ft section of 11" belly heat should adequately heat the whole tub, whereas a strip of heat tape along the short side of the tub would not heat the other side of the tub enough and you'd end up with a spot that was too cold(unless of course you heated the entire room but thats a whole 'nother discussion entirely). So anyways this is just my extendedon this subject and it is entirely based on my opinion and personal experience.