In my area of PA, a heat pump is marginal, but it's cheaper than either propane or electric.

Hello,
What is your budget? Do you have enough for Geo? You must be outside of gas service? How soon will utilities be forced upon you? If you bid geo, do not use the big boys in Dallas, they are too high. Find a smaller contractor.

At this time, it is a toss up as to the cheapest energy. TXU is in trouble, and trying to cram the new deal down our throats. Atmos is just as bad. Between the 2 companies, they set the pace for the various co-op's in the area. If net metering revisions pass, Texas will pay 50% of the installation cost for solar electric. This will probably be iffy for co-ops. You need to check with your co-op if not TXU. Check for thier existing attitude on Net Metering

This is not a simple answer for Dallas area today. One way is safe, and expensive. The otherway is expensive and will continually reduce your energy needs for the next 20+ years. Go to NTREG and ask the same question.

Energy costs are so volatile, who knows what will be "cheaper" in the future, gas or electric ???

You haven't started construction yet, so I would recommend to build the most energy efficient building envelope you can afford. For example, instead of stick built with batt insulation, build with ICF or SIP. Instead of huge aluminum frame windows go with small double or triple pane, fiberglass or vinyl windows. Insulate the roof as good as you can. Make sure the house is as airtight as possible.

I'd use radiant floor heating run by a gas water heater because it is easier to zone than forced air and heats more evenly. For cooling, is there anything feasible other than electric air-conditioning?

If you want to use a conventional system, and you want to spend now to save later, go with dual fuel and zone the system. 16 SEER, two stage heat pump, 90% variable speed furnace, two zones.