How does specification of either electric or gas as the heating source for a thermal fluid heater affect the choice of the fluid itself?

Radiant heaters have higher localized film temperatures than electric heaters because the radiant section of a fired heater (heater tubes that face the burner) has much higher heat flux (BTU/hr-ft2). Electric heaters have uniform heat flux. The higher the heat flux, the greater the film temperature produced in that section of the heater.

All things being equal, you can operate a heat transfer fluid closer to its maximum recommended film temperature in an electric heater than in a gas-fired radiant heater.