Lets look at the differences in the two furnaces:
When you look at a Furnace you will see their ratings measured by their AFEU. (Annual Fuel Utilization Efficiency) Which Represents how much energy your furnace actually uses to heat your home and how much gets wasted. So basically an 80% efficiency furnace only heats 80% where as a 90% heats 90% of your home. While both heat your home that extra 10% really does make a difference in your home and your budget.
Another major difference between the two furnaces is how they are vented. The 80% efficiency furnace has a higher exhaust temp so it must be vented through a chimney via chimney liner. Where as a 90% efficiency furnace has a lower exhaust temp and are normally vented through PVC Pipe and are placed along side your home.
So if you happen to be in need or possibly thinking of switching your furnace don’t make it a horrible experience. 90% of your heat used efficiently, hassle free installation, and lower heating costs or 80% heat used, complicated install, and higher heating bills. This is one of those instances where bigger really is better, for your home and your pockets!