The 240V hookup uses half the amperage and since watts equals ohms times amps times amps, using 240V gives you less wattage lost in the wiring to the heater...1/2 the amps and 1/4 the watts. It also reduces voltage drop since volts is amps time ohms...1/2 the amps and 1/2 the voltage drop which means slightly more output from the heater. Using 240V also balances the load on your service, using 120 might add load to the heavier loaded hot leg or it might add load to the lighter loaded hot leg..without an amp meter, you probably won't know. Balanced loads are always better. As far as cost, you pay by the watts used regardless of 120V or 240V so not having the higher watts load in the wire itself would mean you are getting more heat where you want it and less where you don't want it. Without actually doing the math, I'm not sure if using the 240V would save you money overall on your power bill, but it definitely is more efficient! Using 240V might allow you to use a smaller (less expensive) gauge conductor when wiring it up.
