Air-source and ground-source heat pumps do the same job in the same way. Both move heat rather than making it. Both use a refrigerant, a compressor and a heat exchanger. Both can heat a building, heat water and, in many cases, cool a building in summer.
The real difference is where the heat comes from. One takes it from the outdoor air. The other takes it from the ground, or sometimes from a nearby body of water. That one difference leads to most of the others.
The heat source
Outdoor air changes temperature all the time. It can be mild in the afternoon and below freezing at night.
The ground is much steadier. A little way below the surface it stays at a fairly constant temperature all year, because it stores heat so well.
Efficiency through the year
Heat pump efficiency is measured with the coefficient of performance, or COP. This is the heat you get out divided by the electricity you put in. The smaller the gap between the source temperature and the temperature you want indoors, the better the COP.
An air-source heat pump is very efficient in mild weather. On the coldest days, when you need the most heat, the air is at its coldest, so its efficiency drops.
A ground-source heat pump has a steadier source. Its efficiency stays more even through the winter, which is when it matters most.
Space and installation
An air-source heat pump needs a spot outside for its unit, with good airflow around it. Installation is fairly quick and does not involve digging.
A ground-source heat pump needs a ground loop. That means either trenches across a good area of land or deep boreholes drilled by a specialist. Both take more time and planning, and the work is more disruptive while it is happening.
Because of the groundworks, ground-source systems usually cost more to install.
Noise and appearance
An air-source unit has a fan, so it makes some noise. Modern units are fairly quiet, but placement still matters, especially near windows or neighbours.
With a ground-source system, almost everything is buried or indoors. There is no outdoor fan to see or hear.
Running and maintenance
Both types need very little day-to-day attention. An air-source unit needs its outdoor coil kept clear of leaves and debris. A ground loop, once it is in and tested, is out of sight and should need little attention.
Which is better?
Neither is better in every case. Air-source heat pumps suit most buildings and are simpler to install. Ground-source heat pumps can make sense where there is land or space to drill, where the heating demand is high, or where a quiet, hidden system is important.
The right answer depends on the building, the site, how the space is used and what you want from the system. That is why we start every project with design: looking at heat loss, available space, cost and payback, and how the heat pump will work with solar and batteries.