I'm looking to build a house and so going through the process of understanding the details of the specs. Right now the builder uses an 80% Efficiency model, but others use a 90%. Is the 10% worth the investment to upgrade to (have not asked what that is). What number should I be looking at or what is the "best". Thanks!!

With the ever increasing price of gas I would buy the 90% unit. The cost might be 500.00 or more higher but you will recover the cost in short order if you live where it gets cold for a good part of the winter.

NOTE get the longest warranty for both parts and labor that you can. The parts are not cheap and the labor can also cost big bucks when repair time comes.