AIR source heat pump?

Garry recommends an air source heat pump.
The disadvantages (compared with ground source energy which is a more constant source) would be that it is slightly less efficient in the most cold days and that you couldn't use it to cool the house IF you had a cooling system (which we are not planning to build anyway since it is too complicated for our old house).
But both, air source and ground source heat pumps, would be of same capacity and in either case he would recommend a backup since you couldn't guarantee 100% efficiency all the time but perhaps for 95% of the time.
The advantages of the air souce heat pump would be

• that it is far simpler to install (no drilling and no trenching) and the air and heat exchanger are in one single unit which has to be outside (which is important for us as storage space is scarce)
• that in the summer (with warm air) the production of hot water is much more efficient than from ground source