I have a small room about 88 square feet, it is insulated with r25 on the walls and ceiling, heated above and to one side, and it has a small 32 by 32 inch double pane window in it. one side faces the outside and one an unheated back porch. This room has a 250 Watt heat lamp in it. Because it is so well insulated I believe you could just about keep it warm enough most of the time with just that lamp with the exception of the coldest months when the outside temperature can dip to below zero degrees Fahrenheit where I am at. I don't actually heat this room except when it is in use though, and when the temperature is extremely cold and I worry about freezing - as it has a drain line along the outside facing wall - I open the door and let warmer air in from the adjacent room which has forced air gas heating. This room is actually a bit too well sealed as when closed up for a bit it is actually a bit stuffy.
Most rooms of similar size however will have much more heat and air transfer and would require at least a minimum of double that amount of heat and probably a bit more in cold weather I would think. The nice thing about the infrared is that it disperses the heat and feels warmer on your skin instantly. The bulbs do burn out though, especially when accidentally knocked, and the long term costs of heating this way are therefore likely a bit more than resistance heating.
Heat is transferred by convection, conduction, and radiation only, so eliminating any air leaks, adding insulation and curtains can reduce the heating requirements dramatically, and this is the place to start regardless of what type of heat source you ultimately use.