We replaced our gas furnace last year and chose an 85% unit vs new high efficiency 90-96% side venting on advice of several installers.  They said extra price and efficiency didn't justify the extra markup and complexity/reliability issues of high efficiency units.

We did get a variable speed, low voltage blower that should save some plus it improved the efficiency of our SEER 13 AC purchased the year before. Difficult choices, and the estimates I got for the same equipment were amazingly far apart in price, as much as $1500 for the same furnace.  Unbelievable!

I can swear there ain't no heaven but I pray there ain't no hell. _ Blood Sweat & Tears