I have a 4m x 4m garden office, I prefer the heat from an oil radiator in there, which I have close to where I sit. I think because it gives out radiant and convected heat its a more comfortable heat source.Update:
After trying the cheap fan heater less than £15 but with current UK cold winter, the fan heater was ditched away and oil heater was back, is clear winner in such weather.
The fan heater is OK for summer as has fan function only and not in so cold weathers.
The fan heater is good for instant heat but once off freezing cold.
I have wall fan heater in the bathroom which is good because you don't stay there long and 1KW doesn't cost much for short periods.
I will update as to which is cost effective, I believe will be the oil heater.
But a fan heater warms the overall room much much faster.