[roryirvine]

The main benefit is that air to water systems are compatible with existing central heating systems - but only if the radiators are sized correctly, and often they're not.

If you factor in the need to replace radiators etc, then the ability to act as an air con probably makes air to air the better choice. Now that the Boiler Upgrade Scheme grants cover air to air as well as air to water, I'd expect them to become much more common in the UK than they have been until now.