Let's begin with a basic approach to the difference between ground source and air source heat pumps.

Ground-source heat pumps have pipes in the ground called a ground loop.  That loop is connected to the heat pump equipment which is inside your home and typically in the basement.  Being located inside, it increases the efficiency and life span of the equipment. 

Air-source heat pumps are set outside, on the ground adjacent to your home.  This leaves the equipment exposed to the elements reducing the efficiency and the life span of the equipment.

Are there circumstances where I would recommend an air-source heat pump? Yes.

Do they cost less initially? Yes.

Are they as efficient? Not even close.

Does the increased savings of the ground source make up for the additional up front cost?  Typically in about 5 years.

Starting with the ground-source heat pump(gshp), consider that the system circulates a water solution through the pipes returning the water to the gshp at let's say 50 degrees.  The gshp then needs to increase that temperature through the compressor to reach the temperature you need for heating.  For cooling, the gshp is taking heat from your house and using the 50 degree water to cool the air.

Now take the air-source heat pump.  When you are requiring the most heat, your system is using the outside air and then sending that through the compressor to reach the temperature you need for heating.  For cooling, the air-source heat pump is taking heat from your house and working to 'dump it' to the outside air.

If we think of the earth as a heat bank for the gshp and the air as a heat bank for the ashp, we will see that it is easier to 'take heat' from 50 degree earth than 20 degree air.  It is also easier to deposit or dump heat to 50 degree earth than it is to 80 degree air.

How much difference will you see in the cost of operation?  That will depend on your specific location, site conditions, and cost of electricity.  Only by performing specific analysis can you be given an accurate answer.  When we design a system part of our design package includes a cost of operation for the gshp system and a cost of operation comparison to other systems including air-source heat pumps, oil, natural gas, propane, and electric resistance systems.  We would be happy to provide a free consultation for your application.