I live in Minnesota where our cooling season is very short, about 3 months. I have received bids on both two stage and single stage central AC units (about $2500 difference between the two). Is there a significant difference between a 15.5 SEER single stage and a 21 SEER two stage? I understand the "functional" differences, but will they really run that differently? and will I ever see any return on investment in energy savings if I go with the more expensive unit? It also seems that the higher end models run louder according to the manufacturer's specs. (More money and louder? Hmm.) The contractors naturally want to push the higher end unit, but I wish they would also be realistic about our actual AC usage. Comments appreciated.