The Carrier Infinity is the best furnace from one of the best brands. I believe Armstrong and Airease are brands made by Lennox. These are considered second tier brands. The component and build quality would not be the same as the Carrier Infinity models.

The installation of the furnace is very important. A top of the line furnace will have problems if it is sized and installed incorrectly. Choose the contractor carefully and be suspicious of the lowest bidder.

How important is it that the primary and secondary heat exchanger be stainless steel? Both heat exchangers in the Armstrong and Airease are stainless steel where as carrier only the secondary heat exchanger is stainless steel.

If the heat exchanger has a lifetime parts warranty, then I don't think you should be concerned. Carrier uses an aluminized steel for the primary exchanger. There was a period of time when Carrier furnaces had heat exchanger failures. They must believe there material is better than stainless steel.