No, both systems have the same heat output and will use the same amount of power. This is because electric radiant floor heating systems use resistance heat, meaning that the resistance (Ohms) of the cables are adjusted so that no matter what the voltage is the Watts produced per a square foot are the same.
The only difference between the systems is in amount of amps they draw per a square foot: 120 volt systems draw 1 amp for every 10 square feet while 240 volt systems only draw 0.5 amps for every 10 square feet. This means that a single thermostat can be used to run up to 150 square feet of 120 volt products, but up to 300 square feet of 240 volt products. If the space being heated is 150 square feet or less, there is no advantage to installing a 240 volt system. In fact, using a 240 volt system in such a small room will just waste an extra space in the breaker panel that might be used for something else in the future.