Reducing Heating Costs - Old Brick Apartment, Space Heater vs Central
It depends on the type of central heat you have. Gas is usually cheaper for heat. A heat pump is more efficient until real cold weather then it starts using electric makeup coils. Space heating is less safe than central there are fires every year where a space heater gets moved two close to combustible materials like curtains and furniture. 
A small room or space can be heated by a space heater and reduce cost but with the larger rooms and keeping the majority of rooms warm I would use the central system even if it is straight electric for the safety.