Old thread, but 2 more cents worth;
Imo, people get caught up in the fact that costs to cool with air/air units is fairly low. That's true, but look at it this way: 100 out, cool house to 70 = 30* difference. Zero out, heat house tot 70 = 70* difference.
You can buy an air to air that'll work at -20F or lower, with no add on elec strip "toaster", but I question the logic of that, due to initial cost. I also question the logic of the "geothermal" systems that many people brag on for the same reason. Although, I must admit that a friend of mine has a not that expensive small [2 ton?] air to air mitsubishi that works to -10F with no resistance backup, no lie. I'm not sure of the SEER #, but it has to be fairly high, in the 16 to 18 range, I suspect. I only heat with imy air to air a few weeks during spring and fall, but my 22 year old york 10 SEER would be on the toaster long before that in cold weather. Fyi, that 3 ton York has cost me a total of $140 for a new condensing unit fan motor over 22 years. Then again, it sits idle for 6 months a year when I'm on my coal boiler.
On ductwork, imo: Ideally, for heating: Low supplies, high returns [works ok for cooling]. Ideally for cooling: High supplies, high returns [does not work well at all for heating]. Realize that if your ductwork is not wrapped, it will likely sweat and drip while cooling, esp if it is in a humid area, like an attic, or an unconditioned basement, or esp a crawl space. You will loose a large amount of btu out of uninsulated ductwork that's in a non conditioned area imo, esp when heating.
I'm just a dumb tin banger, but on units: I would avoid those that have variable speed [ramping] fan motors, which are troublesome, and very costly to replace. Imo, esp if you will use coal in very cold weather, something in the 15-16 SEER range is the best bang per buck as far as out of pocket money over the life of the unit. Afaik, the very high SEER units will not save you that much when cooling, vs a lower SEER unit.
p.s. - depending on how old your window units are, even a 13 SEER might save you a little, esp if you can get any rebates.