You were quoted the 15 SEER XL15i air conditioner and the XV80 (variable speed, 2 stage) furnace. The furnace is only 80% efficient, which is important to note. What is your location? Is venting with PVC piping (as required by 90+% efficient furnaces) problematic? If you live in a climate where heat is primary, you may consider switching the air conditioner to a heat pump.

The system does qualify for the tax credit, which a 13 EER and 16 SEER rating.

Size determined by load calculation or just by what is existing? For 4500 sqft, 5 tons wouldn't surprise me. Does the home have just one system? Zoned? Ensuring proper ductwork size and design is key.

I am in Northern Virginia. Is AC or heat pump better for this particular climate?

I think the contractor proposed the new system based on the existing one. The home has only one zone.

I'd say dual fuel is a good option if you have reasonable electric rates and higher gas prices. If you post your avg. rates here, another member here, TD, is very good with analyzing costs and can give you his opinion on which way to go. Heat pumps generally heat more efficiently than gas furnaces in terms of operating costs, but there are other angles to look at the situation from. For one, heat pumps run all year while an A/C runs half the time.

If you use gas and straight A/C, then I would look into upgrading the furnace to the XV95 (95% efficient) furnace.

I totally second the dual fuel recommendation. It shouldn't cost much extra and gives you great flexibility incase prices go up in future. This year since gass is cheap I run my heatpump down to 30 maybe 35 degrees. In Virginia most of your heating would probably be with heatpump.