Greg,
In the case of a dew heater, I believe all of the electrical power is converted into heat regardless of whether it is wire or thin film resistive device. The only difference will be how much of that heat is transferred to the place you want it. If the heater strap is not well insulated, some of the heat will pass to the surrounding air (convection) however if the contact between the strap and the OTA is good, the heat transfer rate from the strap to metal (conduction) will be much greater.
Kendrick also has a webpage discussing power consumption, heat distribution and capability of heater straps with a lower power rating:
http://www.kendrickastro.com/dew_flyheaters.html
I don't believe there will be any advantage in terms of power saving if you are running the strap in a temperature controlled mode.