Maybe that explains why, in high school, every time my friends and I hoofed it over the pass to hit on girls at Kah-Nee-Ta, it was raining...

Once you know your expected seasonal BTU requirements for your house, you can calculate run time, and from there the DSH contribution. As the envelope gets tighter, the proportion of household energy going toward producing DHW increases. Have you considered a hybrid geo system that can produce both forced air and hot water? They exist now, but I understand there's some new ones coming to market targeted directly at your needs.

I had given it some thought but my initial investigation looked like they were for pretty significant use. We just don't use a lot of hot water. Our typical use now is 600 to 700 Cu Ft of gas per month for water heating, with a standard water heater.  We won't have gas for the new home.  I guess when we have the energy Modeling done, and get actual load calcs it will be easier to lock things in.

Last thought - since it sounds like you don't expect much of a cooling load, have you considered radiant? Waterfurnace NSW line is a nice water-to-water unit with output as low as 1.5 tons. Great for tight envelopes, not overkill for domestic hot water. The heat exchanger in these is compatible with potable water. Central humidifying isn't compatible, but from what I understand, if you build a tight envelope lack of humidity usually isn't an issue.

If the marathon costs you $500 after incentives, it sure would be a consideration. If you can double dip, a marathon buffer tank with an ASWH, for $ 1000, sure, go for it!
In my neck of the woods, to are talking twice as much money for the marathon installed, and 3 times as much for the ASWH.
ASWH shine in A/C dominated climat, where they cool and dehumidify your basement.

Actually before reasearch, that was the direction we thought we would go. The issue is, we are designing a lot of Passive solar into it. With the cool nights, and a tight, well insulated home, your radiant heat will typically charge your floor in the early morning hours. Then when the sun comes up, your floor is charged and the passive solar does nothing but overheat the space. 

This information was found in a number of places including local area Architects.
Because the climate is very dry to start with, the humidifying is definitely an issue in my house. ( at least that is what I have been informed!!)

Then when the sun comes up, your floor is charged and the passive solar does nothing but overheat the space.

Radiant floors require constant heat input or they will cool down. Once the sun comes up, you can replace the hydronic input with the solar input. It also helps if you have a ventilation system that can recirculate air from sunspaces.