Two stage units, whether using a single variable speed compressor or dual compressors, have more complex control systems. Some also have variable speed blowers in the outdoor unit. Typically, two stage units are matched to variable speed air handlers, again with more complex controls. Every thing else equal, added complexity always decreases reliability. The real question is, "To what degree?" For that one, the jury is still out on the latest units. We probably won't know for sure for several years.

tedkat you may and I emphasize may achieve better floor to floor temp constancy with a two stg condenser running on low stage for longer run times. However, I believe floor to floor temp constancy is more a function of good insulation qualities and properly sized ductwork design both supplies and particularly adequate returns. IMO

Yes, 2nd floor air handler is in the attic. 2nd floor is around 1800sqft (but large chuck of that is open space about the family room and foyer). The air intake for 2nd fl unit is just in this big open hallway area. The original unit (Trane XE1000) is 2.5 ton, and we are being recommended the same size. The 2-stage one (XR17), it doesn't come in 1/2 sizes, so we'd do a 3 ton in that case. For any of the siingle-stage ones (XR14, XR15, XR16) we are being quoted 2.5 ton. It sounds like contractor is doubtful that we'd notice much of a difference using a 2-stage. If that is the case, then I would probably just go for the cheapest XR14 with var speed air handler. I am willing to spend the extra, if I would really notice a difference, then maybe its worth the extra +44% to go to XL17. But that's a big jump in price if there is negligible difference. Because we have such a big open area which combines with downstairs air -- will I notice a difference? I can't recall the AC not being able to keep up. The one thing that does annoy me about this design, is that the thermostat is out in the "open" area. So in the winter, if we turn down the heat downstairs, and people close their bedroom doors, then the thermostat ends up being much cooler than the rooms, and temps are all uneven. I think that is a different problem, and not affected at all by which unit I pick.

Propane has 91K btu's per gallon input, Electric has 3400 Btu's per kilowatt. Propane gives you 72,800 btu's output at 80% efficiency. Electric aprox. 3400 Btu's output per kilowatt, (100%) with only a small amount lost. 72,800 btu's divided by 3400 btu's = 21.4 kilowatts to equal 1 gallon of propane. The average cost of 1 kilowatt in Austin - $0.1142 cents, times 21.4 Kilowatts equals $2.42 for electricty and little more expensive then propane at $2+ per gallon. But this will change drastically if Biden does ban fracking as he stated. But then what happens to the price of electricity? It's a guess, and having the ability to switch between the two may be a valuable option in the near future. But the cheapest option with a stable price for ever would be to capture the heat in the sun with active solar collectors to heat water (not to be confused with solar panels to produce electricity), and store it in insulated underground tanks to use for the winter. After the initial investment it would cost you a few dollars a month in electricity to run circulation pumps, and you could even get the electricity from solar panels for that. They do it in Alaska for building bigger than yours, easy and simple, only problem is getting insulated storage tanks in the 5K to 20K gallon range. I'd do it in a heart beat if I had some really big tanks, I have the land, backhoe and the solar collectors, 9 of them 5' x 9', salvaged from a $35,000 government subsidized program project in the late 70's. (The government loves wasting other peoples money.) Johnson controls started a line of HVAC equipment called champion about 6 years ago. Parts?