Belly heat from a mat is preferable, I think. I use mats for all of mine. Corns need heat beneath them to digest. A lamp would provide overhead heat and a hot spot, but as Corns don't bask, this may not be ideal.
Heat lamps keep the ambient temperature in the tank up, but as the Corn lives at ground level (unless you provide a lot of climbing equipment), heating the air above it seems a bit pointless.
Whatever heat source you choose, remember to use a thermostat to control the temperature. An unregulated mat or lamp will easily get hot enough to cause burns.