Geothermal energy or water-to-water heat pump?

Hello! In the region around Berlin, both variants of heat pumps are particularly recommended (after some consulting discussions). The question now is, from your neutral perspective, which is the better choice. We would like to use the heat pump exclusively for heating and hot water without additional devices (so, e.g., no separate device for hot water) for 250 m² of usable space (the basement is to be partially heated). Is that even realistic? There are very contradictory statements about this. One says you should only use the heat pump for underfloor heating and install an additional gas condensing boiler for hot water (which I personally consider complete nonsense), another says it is no problem at all. However, the latter must also be viewed skeptically because everything has to be calculated precisely. As I said, my main concern is the basic question: geothermal or water/water heat pump (both with deep drilling).

Goal: supply underfloor heating on about 250 m² and produce hot water for 2 adults and 2 children (the children are still in progress ;-))

Hello,

.... In the region around Berlin, both types of heat pumps are particularly recommended (after several consultation talks).

Who did the consulting? ;-)

....One says you should only use the heat pump for underfloor heating and install an additional gas condensing boiler for hot water (which I personally consider complete nonsense), the other says it is no problem at all.

Both are right, but with quite different orientations ;-) The fact is that preparing hot water with a heat pump is inherently less efficient, because a higher flow temperature is required compared to the precise sizing of the heating surfaces (heating operation). The annual performance factor for hot water is always lower than that for heating operation, assuming the latter is properly dimensioned and planned. But: In many cases, the energy expenditure, not power, for hot water preparation is about one-third of what is required for heating operation, depending on the Ht value of the building envelope. Thus, the reduction in overall annual performance factor usually remains, not always, within limits.

....Although the latter also must be viewed skeptically again, since everything must be calculated precisely.

Correct!

....As I said, my concern is initially the fundamental question of geothermal or water/water heat pump (both with deep drilling).

Water/water heat pumps can theoretically achieve higher annual performance factors than brine heat pumps because the source can provide a higher temperature level. However, everything must be right here, especially the water quality (fouling of the feed and/or injection wells). Many projects fail because of this. In the COP specifications of manufacturers according to 14511, the energy expenditure for the source pump must be included. One doesn't have to be a fortune teller to recognize that the boundary conditions are deliberately optimized here to achieve the high COP values. In practice, these are rarely reached, mostly due to water quality or hydraulics reasons. A risk the builder has to assume ;-)
With brine heat pumps, there is the possibility to use various sources. In addition to the typical vertical borehole(s)—several are better than one but unfortunately also more expensive ;-)—surface or trench collectors can be used. For the latter, the high effort for source development can be significantly reduced by own work.
Ultimately, it depends on how high the actual demand for heating and hot water is. The higher it is, the more necessary it is to achieve a high annual performance factor; the lower it is, the less economically noticeable the annual performance factor of the heat pump solution is.
In very well-insulated buildings, digging or drilling into the ground is unnecessary; here, a fully modulating air heat pump is sufficient.

Best regards