how long are you planning to be in the house, the higher efficiency the furnace the better it will be on your bills, but depending on climate and usage it may be some time before you get a return, and then its not really proven how much a high efficiency means in terms of resale value.
have you also looked into carrier, lennox, trane and york?
here is one site, its not that great but gives you some general comparison, you may want to look at cinsumer reports as well.
yes I've frantically checked out the consumerreview and it has helped me a little in understanding
however, even if we go for the best one that may cost $3500 or above - how do we make an estimate on how much it will cost us per month in bills.... that's pretty tough.
get an average of how much you spend in heating and how much gas you are using and your old furnace efficiency ratinngs and the new ones. then you can calculate based on diff gas price scenarios