Should I use a 120V or 240V electric floor heating system?
As a rule of thumb, when you are heating an area over 130 sq ft, you will likely use a 240v system(s). Why?
120v systems provide more options for covering smaller areas; similarly, 240v systems provide more options for larger areas.
Whether you are using one or several heating systems, each specially designed thermostat that will power them can handle a load of 15 amps. Anything more than that and you will need to purchase an additional thermostat.
The 240v products use less amperage which might help if you have limited space/breakers in your electrical panel.
240v systems can also be used as a primary heating source. Therefore, if that is your goal, be sure to consider purchasing a 240v system.
The bottom line: The more heating systems and thermostats you need to purchase will affect the total cost. If you tried to heat a large area with 120v systems, you would need more material and therefore more thermostats to power them. However, this voltage but would be ideal for smaller areas.