What are the advantages and disadvantages of the different types of heat pumps?

There are debates over which system is the most efficient however, an air source heat pump is reliant on the outside air temperature which can be a lot cooler than the ground temperature which by comparison is relatively stable. Consequently, a ground source heat pump tends to be more efficient in all outside temperature situations however, they are significantly more expensive to install and are therefore less popular than an air source heat pump.