Since you are considering a Daikin Altherma system, I will use that setup for a comparison here.
The units all run with R-410a refrigerant, which is the current American standard. Unfortunately, it doesn't do so well in high-temperature heating applications, due to its somewhat low critical temperature. Above about 110-120 degrees F, heating performance drops pretty quick the higher your destination temperature is lifted. The units all have been designed to yield a COP of about 4 into a 55 degC destination (heat sink) temperature. If you can live with a 50 - 55 degC heating water temperature, the low-temp line looks pretty attractive.
However, if you need higher heating water temperature (due to radiators, baseboard heaters, etc.), the COP of the Altherma units drops off to below 3 with the additional heating gradient. I didn't look deep into the ratings, but I'm sure those figures are not in frigid outdoor air temps. As outdoor air temps drop close to and below freezing (0 degC), the unit will defrost more often, further cutting into efficiency and raw heat output. Combined with the high-temperature heating gradient, this makes the heat pump not very efficient. Economically, natural gas is a cheaper heat source during frigid weather (more so when high-temp heating).
For the reasons stated above, I would definitely look carefully both at your system design and supplemental heat sources. If you only need high-temp tap water, a modest solar (thermal or PV) source could boost that small percentage of total demand with ease. You could use a low-temp heat pump do supply most of the demand in its efficient operating range. OTOH, if you need more high-temp water to supply space heat, there are trade-offs to be made. There is a big difference in outdoor unit expense between a unit that can provide total capacity on a cold day (with excess capacity the rest of the year) and a unit that (only) provides capacity 90% of the time. The difference in upfront cost could take a decade to pay back in efficiency.
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