they both dont produce very much heat to begin with. and your right the better tech "should" produce less heat or at least more manageable heat. to be honest i doubt anybody is going to be able to give you an exact answer unless you have a laser thermo and both side by side.
but the atom doesn't have any thermal monitoring and has very limited throttling capabilites. the celeron can throttle itself much more efficient and thus when in a low power state will put off less heat than the atom. when under load it may produce more... but over time it should in theory be better suited.
but my guess is your going to want to go with the celeron anyways because that atom is pretty old and lacks in the newer instruction sets, doesnt incorporate intel hd graphics, and it only supports ddr3-800. not to mention the celeron is at the least more than double the speed of atom. i cant imagine a situation where the small differences in heat you "may or may not" see between the two would be worth sacrificing all the advantages of the celeron.