But think about an partly cloudy day. The rock they've found to bask on will retain a good portion of the heat even if the sun is behind the clouds for 5 minutes. Overall though, I would think that 90F, for example is 90F anyway they can find it, i.e.; shade, sun, rock, stump and etc. I don't think they would discriminate one type of heat over the other. I think they would just find the proper temperature to achieve what they need to achieve.
I've built racks with heat under the bins = belly heat, and racks with heat up the back = ambient heat. I must say that I feel more comfortable offering the belly heat but my more logical side says "it's the same either way as long the proper temps are offered."