That is a crazy high gas rate. Do you have a flat fee worked into that? We pay $10 a month flat so if you look at what 1 therm costs, it is $11. But truly the therm costs $1. A NG furnace will always save a few in a dual fuel situation but not enough to make it worth it for you. Where is your MBR? Don't you really just want the MBR at 66 at night? What you probably want is more tonnage which isn't going to work with your existing ductwork most likely. So it is pretty cost prohibitive compared to other options. The other options are to reduce your heat load. Air sealing is cheap enough to do some things DIY - do you have attic stairs? Do you have cans into the attic? Sealing these things may lower your temp enough. Controlling solar gain is really where the real savings is. The cheapest is solar screens but they do effect the transmission of light. Plant trees. If you don't have low SHGC windows this can make a huge difference. Switch to LED lights - anything used over an hour a day is a reasonable payback and it will lower the a/c load. LED is far better than CFLs and we find it acceptable everywhere except the bathroom. We keep our master at 65 at night (and high 50s in the winter) with a ceiling fan so I understand where you are coming from. Since your usage is so high, it would definitely pay to get some high efficiency equipment. But overall, the best answer to a cooler house in the summer is not to change the a/c size but to control the loads. I do feel like your usage is pretty high even given your locale and requirements. I peaked at $150 (probably $90 in a/c) in July this year with a 65 degree bedroom suite for 5000 sqft in NC. Obviously you are warmer but not double.

It's nice to have the dual fuel capability, especially since energy rates seem to fluctuate between gas and electricity in many areas of the country. This allows you to set your balance points to whatever fuel is cheapest to run during the heating season. I have dual fuel in my new home but because of my electric rates, I don't use much gas, Other parts of the country are different. A heat pump will run year around because it heats during the fall/winter/spring and the A/C side of it runs during the summer. However, my last unit ran 20 years and during that time we had some unpleasant summers here in the northwest when the A/C started in May and ran through September. I don't think it's a big worry. Amana is supposed to be Goodman's top of the line and the specs on those units are pretty good. Depends on how it's matched with your air handler/furnace.

I'm not sure how a modern heat pump compares to a high efficiency furnace running cost wise, but I would imagine that a properly running one ought to cost less to run than just a furnace and you have the added advantage of electricity being able to be generated a number of ways versus your furnace runs on gas or gas you have no choice (I'm thinking along the lines of the price of oil/natural gas vs electricity) Don't forget that a heat pump has the advantage of being able to take advantage of 'free' energy out there (existing heat in the outside air) which raises its efficiency quite a lot, people will argue, for example that a heat pump is "300% efficient" on the face of it impossible, they mean you get three times the heat for the same amount of electricity than if you used the electricity to just generate heat through heating elements. What you've described is confusing, more details if you can please.

Based on the information you've provided, sounds like the York quote is the best option, but I don't agree with the 80% modulating furnace. That's a high-end furnace, and going with the 80% version doesn't make sense to me, especially in our region where it can get cold at times (I'm in Maryland). Is PVC venting an issue? The York Affinity is a nice line of equipment. American Standard isn't bad, either, but the equipment that the York rep. is quoting is better in my opinion. 3 tons is a bit much for your cooling load, but it's what you'd have to go with if you went with the Affinity line. The 2.5-ton Latitude unit would be the better fit. With the low stage on the Affinity approximately meeting the cooling load at design conditions (which won't be met on all days), I don't think you'll be getting those long run times that you should with a 2-stage system. Ask the York rep. if he can quote the CZB or YZB 2.5-ton single-stage Affinity unit with a 90+ efficient v/s furnace (modulating or 2-stage). Not sure where the American Standard rep. came up with his sizing. Do you like it cold in your house (68-70)? Did the York rep. do the load calculation?