It's comfort feeling. 2 stage will provide less temp delta. However, if your single stage hi-eff is already equipped with a variable speed blower which should be good enough. $2500 seems far too much....
Usually there shoudl be Energystar mark for Hi eff. These days, a variable speed blower will be good.
I have a 10 years old Hi-eff one and it was broken down and had it fixed myself, learnt a lot while searching the internet.
I would highly recommend you to look for one with the variable speed blower, it will be much quiet as well as saving you a lot of electicity. Normal 2-4speed blower will be running at 400-600W while variable DC motor will be about 80-120W.
