A floor heating system operating on 240VAC is not more energy efficient than a system operating on 120VAC. This is a very common "Urban Myth". The power (watts) consumed to heat an area is the product of voltage, amperage, and the resistance of the heating cable. If the voltage doubles, the amp draw decreases by 1/2. So, it requires 1/2 as many amps to operate the same load at 240VAC as compared to 120VAC.
OHMS Law states: P(watts)=V(volts) x V(volts) divided by R(resistance). P (power)= watts, V=Voltage, and R=Resistance. By applying Ohms Law, one can see that a product uses 1/2 of the amps at 240VAC as with 120VAC. The electric company charges for power (watts), NOT amps.
Different voltage rolls are manufactured with different Ohms resistances. For an example, a 60' floor heating systen in 120 volt uses 1350 watts. A 60' floor heating system in 240 volt uses 1350 watts. They are both the same square footage and therefore they both use the same number of watts.
WarmlyYours thermostat and controls have a limit of 15 amps switching ability in either 120VAC or 240VAC. With that limit of 15 amps, the upside of 240VAC is that it allows double the coverage of a 120VAC line. The downside of 240VAC is that it will require a double-pole breaker in the breaker panel, and in many homes the breaker panel is pretty full and it might not have two empty slots.
Once again, if the heated area is 100 square feet, it will cost the same to heat that area using either 120VAC or 240VAC. Both voltages use the same number of watts to heat that area. The electric company charges for power (Watts), not amps.