If you rest the lamp on the tub it will probably make the tub to hot. Having the lamp on a stand or somehow hanging over the plastic lid will still allow heat to warm the tub.
You can use both. I keep a cold home and have a uth, ceramic heat fixture, and day and night heat bulbs.
What is the temp of the room you will have him? I need to know to tell you my best suggestion.
This is my forty gallon tank with the heat sources on stands.
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A uth that is 1/3 the size of the tub floor is common.