I personally think the under tank heater works best as long as you have it controlled with a thermostat or dimmer. The heat lamps will cause your humidity to disappear and I don't trust "hot rocks". I am always concerned that they will short out and burn my reptile. If you are having issues with the ambient temps you could always use a small space heater to increase the ambient temps. That is my $0.02 anyway.

Good Luck!