Heating decision: air-to-air or air-to-water heat pump

I would always tend towards a water-based system. If at some point in the lifetime of the place there is a great new heating technology (Wasserstoff, Fluxkompensator, whatever), you are more flexible with it.

I would always tend towards water-based heating. Should there ever be a great new heating technology during the lifetime of the house (hydrogen, flux capacitor, whatever), you are more flexible with it

Yep. And until then, I actually find the heat pump optimal: In my decision, it also played a role that I can generate the electricity for the heat pump by nuclear/coal/gas/wind/sun/fusion reactor/flux capacitor/whatever.

Thanks for the input :) Apart from the fact that it doesn't necessarily have to be company A or B, the air-to-air heat pump was used in a KFW55 house and the air-to-water heat pump in a "climate-friendly residential building – with QNG," meaning KFW 40+? At the moment, we would probably choose the highest KFW level, also because solar panels are going to be installed anyway. I’m going to read up on some facts about 'Proxon' ventilation systems now :)

also because solar is going to be installed anyway.

Recommendation: Differentiate between solar thermal and photovoltaic. Often the former is said and the latter is meant.

Recommendation: Differentiate between solar thermal energy and photovoltaics.

Sorry, thanks - photovoltaics is meant :)

All good, the solar installer does that anyway ;) you just do yourself a big favor if you use the right terms from the start for yourself.