Most all forms of heating are near 100% efficient, since it’s the waste heat you want. Unless the central heating is using a heat pump instead. Does your central heating use gas heating? If so, using it will probably be cheaper. If it uses resistive heating, the individual unit might be cheaper. But if it uses a heat pump, it might be cheaper to use central again. There are a lot of variables it’s hard to know.
Okay, that’s resistive heating. So it’ll be the same efficiency as a oil heater or any space heater. So heating less space with it will save money.