94 AFUE is 14 percent more efficient than the 80 AFUE. If your annual gas bill for heat is $2000, you will save $280 the first year alone. In ten years that will amount to $2800, if gas prices do not skyrocket. If the high efficiency 94 AFUE furnace costs $1000 more than the mid efficiency 80 AFUE, you will recoup that money in 4 years. You will also need less gas, should supplies become tight. All in all, I cannot see any negatives except the extra money needed upfront to purchase the more expensive 94AFUe furnace.