"Green" Heaters ...
As far as I'm aware, in the U.S. "green" is an advertising term and has no actual meaning/weight/value.
Again, AFAIK, a given amount of electricity produces a given amount of heat. E.g. 1500 watts @ 120v produces 5120 BTU's of heat. No electric heating system is inherently more efficient than any other. That said, the design could have a major effect on real world perceived performance.
My mom has a large, glassed in porch that is well-insulated but has no heat. A 1500watt heater will raise the temp by about 10F per hour. A heater blowing hot air makes the room feel warmer more quickly, but if the outside door opens the heat feels 'lost' and the heater kicks on again. An electric, oil-filled radiator producing the same heat (5120 BTUH's) seems to take longer to heat the room, but once the room is warm, a person opening the outside door causes less of a perceived heat loss. Why? The radiator is still hot and producing heat, even though the unit is off.
In the U.S. (and all countries with cold winters, I assume) there are ways of calculating the amount of heat loss a given room has and figuring out how many BTUH's of heat are necessary to keep it comfortable. I believe the 'design temperature' in NYC is 0F/-18C for buildings. This means the heating system needs to be able to keep a room at a given temperature (68F?) when the outside temperature is 0F.
Did this clarify anything?
Jim