I don't see the decision. An 80% furnace wastes 20% of the fuel put into it. A 90% and higher uses at least 10% more of the fuel. The gas savings will more than pay the difference in cost over time.
The 90+% furnaces are where the manufacturers are putting their extra efforts and advancements. The 80% units are the low end products with good reason.
I recommend a 90+% sealed combustion unit that uses outside air for the combustion. When the burners are operating they use a considerable amount of air for the combustion. That creates a negative pressure situation in your home. That is the cause of many drafts.
Have a quality HVAC contractor do your job. Trying to short change this will haunt you for the life of this unit. Have a top of the line air cleaner or filter installed. You might want a humidifier also. A properly sized and installed unit will perform the function they are designed for with the least amount of interference of your enviorment. To translate...a furnace doesn't have to sound like a jet engine to heat your home.
One last thing, your brother in laws friend Clyde probably doesn't have the expertise to give you a good system. I don't care how cheap he is.
Another last thing, I would not buy a service contract. They rarely pay for them selves. Have the contractor service your unit on a yearly basis and change or clean your filters monthly.