I'll just mention that with electric heaters, Watts is Watts. Those Watts may be in the form of hot air (convective), glow (radiant heat), or often some combination, but no electric heater is more or less efficient than another. All of the electricity consumed winds up as Watts of heat in the room. One type of heater that I've used in large areas was a radiant heater, some with parabolic reflectors. These provide a infrared light beam that heats whatever it hits, but not the air, so you feel the warmth but the volume of the room isn't heated very much. This works fairly well to keep warm when working on something, and if you direct it to "shine" on your bikes they will get warm. It's still pretty expensive to run.

One trick we used to use was to cover equipment with a tarp and put a 100W light incandescent drop light in with it. The 100W was enough to reduce relative humidity and prevent condensation and help dry stuff off.

Of course the high-dollar solution is to insulate the garage and garage door and either extend your house's heat system or install another in the garage, perhaps even a pellet stove?