Air to Air is widely used in residential property in Sweden but they have relatively cheap electricity there due to hydro etc. Its performance does vary with outside temperatures but I would suggest if there can get it to work in Sweden it will work here. I have never looked at it in detail but CoP of 4 sounds quite optimistic. But as others have said, if mains gas is available, it will be the best option financially as generally you need to hit a CoP of more than 4 before any run cost savings are achieved, let alone the repayment of the additional upfront expenditure. Bear is right that they are more efficient (and cheaper) if they are producing at a relatively low temperature which is all that is required in a well insulated home or building.
If you are looking at ground source, CoPs are higher and more consistent but installation cost is higher. There are a number of reasons why they work very well in the commercial sector. First, they can provide cooling as well as heating where they are replacing existing electricity usage. Secondly, they can be used to recondition heat that is being emitted as a result of other processes in the building, plus of course there is the subsidy in the form of the RHI. Subsidy for Ground Source can be higher than Biomass dependent upon total energy use. The other benefits of Ground Source is that the installation should last a lot longer than a boiler. Loops should be good or 50 years and heat pumps for 20. Maintenance costs should also be less but this is not yet the case in the UK, but is in countries with greater adoption rates such as the US.
As regards how clean is electricity to gas, the ratio of CO2 produced per 1 kWh is approximately 2.5 times so providing you have a CoP of more than 2.5 times the CoP of the gas boiler, you are saving carbon.
Biomass has its place and I don't see local producers of pellets being effected by huge requirements of the Biomass Power Stations in the UK, Denmark etc. These guys are only interested in people who can provide significant levels of fuel which rules out most local producers in the UK and therefore I can't see a huge impact on their prices where I imagine logistics of delivery is the key driver.