If it is a normal fireplace? If so, most of the heat goes right up the chimney.
Even if it has some sort of device in it to radiate heat, the heater is going to do a better job of evenly heating your house.
Are you trying to heat the whole house, or just the room the fireplace is in?
And is the heater gas or electric?
All factors, but normally the heater is going to be cheaper.