ᐅ Ground Source Heat Pump vs. Air Source Heat Pump

Hello forum members,

My girlfriend and I are currently planning our house construction. Last week we received the final plan from our builder. Now it’s about the heating system.
We don’t have a heating load calculation yet, but I wanted to start looking into the heating topic because it’s very important to me.
You will quickly realize that I am quite a beginner when it comes to heating technology, so I am turning to you here. Please be a bit patient if I bring up the topic for the 1000th time or seem clueless.

Now to the basic data for our house:
- approximately 200 sqm (2150 sq ft) living area
- 900 sqm (9700 sq ft) plot of land
- no basement (a boiler room is planned)
- the goal is to achieve KfW Efficiency House 70 standard
- a tiled stove will be installed (for cozy/visual/personal reasons and also possibly for additional heating)

As I said, we don’t have a heating load calculation yet.
Our builder recommends an air-to-water heat pump with an integrated ventilation system from Stiebel Eltron.
Honestly, I find an integrated ventilation system quite appealing since we both work and can hardly ventilate during the day. I know we’re not building a passive house, but I think a ventilation system is still not a bad idea.

Does anyone have experience with such dual systems?
Are there integrated ventilation systems that can also humidify and dehumidify?
I’m thinking that if the ventilation system can’t do this, it might cause problems for the respiratory system, etc. In the end, we might have issues due to a lack of humidification and be stuck in a bad situation.
Would placing a small decorative fountain inside the house help in such a case?

We have already decided to go for a heat pump. Originally, we wanted a ground-source heat pump with horizontal collectors (the plot size should be sufficient). Through our builder, I have also looked a bit into air-source heat pumps.
He said we would need a heat pump with a heating capacity of 8-9 kW.
That seems a bit too much to me. Wouldn’t 6-7 kW be sufficient?

I am now totally torn between air-source and ground-source heat pumps...
It’s clear that ground-source is considerably more expensive upfront. At Stiebel Eltron, the ground-source heat pump for heating and domestic hot water including ventilation costs roughly €8,000, similar to the air-source heat pump. However, ground-source is more powerful and cheaper to run in terms of electricity. Although the increasingly mild winters also have a positive effect on the electricity consumption of the air-to-water heat pump.
Both should fit into our budget, but will either be cost-effective?
Would it possibly make sense to separate domestic hot water heating from the main system and instead install two collectors on the roof?

What were your reasons for choosing air-source or ground-source heat pumps?
Please help me with this decision, otherwise I might burn myself here.

Many thanks in advance