ANY electric heater (except for a heat pump is 100% efficient.THey can heat you in two ways: by direct radiation (that's where you face the glowing element or bulb)or by warming the airand blowing it around the room. Most do a combination of both modes.
For a predominantly radiant heater, you want the physically pargest size you can find---this minimizes hot spots. For a convection heater, you want the one that moves the most air. Brands don't make much difference.