Just a homeowner here... A geothermal (or any other type) system will maintain desired temp as long as it is sized and installed properly. The first step is to get a load calculation done. From there you can start to figure out if it's the most economical type of HVAC to go with, if that's important to you. My GSHP maintained my set temp in my 1954 house in NY down to lows around 0F.

A properly designed and installed ground sourced heat pump (aka geothermal) will keep the house at the desired temperature. In the winter the system will need a back up source for heating when it can't deliver enough heat. That can be a fossil fuel or resistance heating.

Will you have access to natural gas? If the answer is yes, then most likely it will be cheaper to heat the house with gas. In the summer the geothermal should be less to operate, but the an air sourced inverter heat pump come will come close in operating costs.

If you are going to go the geothermal route, then you have to find an installer that you are confident who know what they are doing. Don't rely on your general contractor to do this for you. The typical GC knows very little about HVAC and is going to subcontract the work to the lowest bidder. I would ask for references of former customers before spending the money on a geothermal system.

Who did the manual J load calculations and what are the water sources? Get the room by room HVAC load calculations. That is step one in determining your options.

Actually water source is more efficient. The heat transfer temps are more stable. Aiir to air efficiency drops with the outdoor air temperature do to more defrost cycles.

Our water source is ground well.

As long as you are not paying for the water.

The load calculation needs to be done and owned by the company that does the installation. If the company can't or won't do the calculation, then you should not hire them.

Presuming load calculations say we will benefit from it and we use an excellent installer - with those presumptions in mind is there a compelling case to do a geo system? I have been told the traditional HVAC systems have come so far that they can offer the same results (with proper insulation) as a geothermal system can - and cost less for installation for sure.

“How certain can I be that a Geo system will really perform
as well with regard to maintaining temperature in the house?”

Properly designed and installed, the answer is 100% certain.

Keep in mind that if you have not removed the original cast
iron radiators that geothermal can work with these as well. That it’s also
possible to incorporate air-conditioning with geothermal in such a system. You
can have central hydronic heating and cooling without ever having to install
ductwork so that the heritage look, feel and value are maintained while having
the most modern high-tech controls and uniform comfort.

IMPO

SR

" is there a compelling case to do a geo system? "

In some situations, yes, in many more situations, no. Any system designed and installed by a competent contractor will meet your needs and provide the temperature comfort you desire, no matter what kind of equipment is used. The differences among the different choices that you need to address are acquisition costs and operating cost.

If you have natural gas service, a ground source (geothermal) heat pump will be considerably more expensive to buy/install and more expensive to operate. . If you don't, some kind of heat pump system (whether air or ground source) could be a good choice.

You're not going to get definitive guidance here. If you have an excellent contractor as you say, ask them to do the technical assessment and then present fully costed out alternatives, including projected usage costs over the course of a year. That information will help guide your decision.

Good luck

We put in Geothermal when we built our rural house in 2009. At that time, the Govt credits and electric company rebates were fantastic, and paid for something like 30-50% of the cost, if I remember right. In those circumstances, I feel it was the right choice. It works wonderfully, is quiet as can be, and little maintenance. We live far out in the country, had plenty of space for the wells, and would have had to rely on electricity or propane, no access to natural gas. We also heat with a woodstove, with wood I cut. So, we were in a perfect scenario to take advantage.

In town, without tax credits, or with access to affordable natural gas, etc I'd examine very hard before biting off the upfront cost of geothermal.