I know you may not know the answer to this question, but I'll ask it anyway. Why the increase in size from 3.5 ton to 4 ton? Sometimes air conditioners are undersized, but more often it is an issue of poor ductwork and air distribution causing comfort issues. Increasing tonnage in this case would then be bad unless deemed necessary by a proper heat load calculation. If you heat with propane, and it's expensive, you will not be too happy with the price to heat with straight electric heat. Heat pumps heat more efficiently but do carry a higher price tag. If you heat for at least 2 months out of the year, I would strongly look into it. By my statement about comfort and efficiency, I mean that there are higher efficiency options, which you were quoted (14.5 SEER) despite what I feel is an unreasonable upcharge compared to actual equipment cost difference. However again I do not know whether your indoor model is changing with that 14.5 SEER upgrade. Which brings me to my next point, variable speed indoor units can raise efficiency while also increasing comfort because they can provide a more precise airflow by adjusting RPM to overcome duct static issues, can be set up to control humidity, and are generally quieter, especially if you run the fan during the day. I would recommend a variable speed air handler with any system. For York, model would be the MV16C/MV20D and that would require a coil as well, FC48C/FC60D, for instance (2-piece set up). The AHP is a bare bones air handler. Nothing wrong with that, if you are looking at upfront cost versus added comfort and efficiency. 2-stage equipment is another option at your disposal and can help with humidity control by running at a lower capacity most of the time for longer periods of time. Doesn't seem like all your options were explained to you by this dealer. I like York equipment, and I have no immediate problems with what you were quoted, but York offers better options in terms of comfort and efficiency. Cooling season is big for you, so higher efficiency and 2-stage units might be worth looking into. Don't be afraid to get 2 or 3 more estimates until you feel comfortable. Carrier is also a good brand to look into with their "Infinity" system.

The Luxaire/York model is the Acclimate HLB030F1

I live in the Seattle area as well, and I was considering the same thing last year. I went with Heat Pump and 80% for a variety of reasons, but the main one is that relatively speaking, our electricity is cheap and you have more redundancy with Heat Pump and 80% furnace. 95% of the year, the heat pump gives me enough heat to keep my house at 71 degrees 24/7. When it gets below 32 degrees or so outside (which isn't very often), the furnace kicks in to supply heat. If the heat pump ever fails, you can use the furnace for heat at any temp. And since 95% of the time, you are running on Heat pump anyways, the gas usage of the 80% eff furnace isn't going to matter much. Remember that because a heat pump gives you MORE heat energy than you put into it (by moving heat from the outside to inside), it is over 100% efficient! No gas furnace can make the same claim. I went with a Bryant Evolution system though and not any of the brands you mentioned. Their evolution control is the best in my opinion and I have been totally happy with it. Whichever way you go though, you do definitely need to make sure you have enough air return. No matter how good the system, an insufficient return will cause all sorts of problems.

Every brand has different levels of systems. All have their garbage models and most have nice dependable high end line. Try and look for a contractor dealer that you feel will do the best job and take care of your mom if she needs some service down the road. Look for systems that meet the tax credit, these are more dependable systems and of course more efficient. Also look to see if the system is a quiet system. The good ones will have a db rating. If it's noisy it's junk to me. Get 3 estimates, do not just look at price, look at dependability of the company. a 100 bucks saved could mean 10 yrs of hell until you get it replaced. Normal span of a quality heat pump is 14 to 16 years if you have it serviced. Some may even work for 20 yrs but most the time 16yrs is max.