Geothermal heat pumps can produce 3, 4 or 5 times more kWh of heat than the electricity they use (this ratio is 1/1 for common electric heaters, using electric-resistance elements).
Air source heat pumps do not provide the same level of electricity savings. Typically, they are able to extract 1 ½- 2 (exceptionally 3) times more heat than the electricity they use, but you will have to resort to other sort or equipment for very low temperatures (or to they electric-resistance elements of the heat-pumps themselves).
Limitations of air-source heat pumps
Recently introduced air-source heat pumps are a lot better at this level. They only become ineffective at very low temperature (say, below -13ºF/25ºC, instead of the 30ºF of their ancestors).
Contrary to traditional heat-pumps, new cold-climate heat pumps provide significant energy savings even in freezing temperatures, allowing a lower carbon footprint.
Anyway, the fact that they still resort to electric-resistance elements on very harsh weather conditions increases peak power demand problems, forcing utility companies to build new coal and nuclear power stations...
Why ground source heat pumps may not be eco-friendly
Unfortunately, geothermal heat pumps are not immune from criticism, either.
They only are really interesting for large buildings and projects. They aren't advantageous from the perspective of the individual household.
Their prices are very high for average residential applications: $30.000 and $50.000.... And that raises a question: isn't it possible to devise a more environmentally-friendly home heating (and cooling) solution with a lower investment?
There are in fact several alternatives, and one stands above all, especially if you are going to build a new home or planning a major remodel: very high levels of air sealing and insulation all over the boundary of your home, and high-performance windows combined with small mechanical heating/cooling systems (and with ductless heat pump systems).
That’s the best low-energy approach, and also the most eco-friendly.