Although the payback time might be a long time, $300 isn't a very expensive upgrade cost. Its often nice to have nicer equipment of higher efficiency equipment.

Usually higher efficiency condenser incorporate additional features which made improve comfort and reliability.

Depending on your installation and equipment you may very additional savings by moving up to a 16 SEER unit.

Do you have the model numbers of the equipment you are considering?

Thanks for the feedback. The condensor unit is the same for both options: Bryant 116BMA048, but the efficiency boost comes with a different furnace: either the 310JAV090048 or the variable speed fan on the 313JAV090048.

I'm leaning toward the higher efficiency unit.

A fixed speed blower will always run at the same speed regardless of the static pressure. But a variable speed blower will increase the RPMs in order to maintain a constant air flow. If you have a duct work issues or a dirty filter, the amount of energy used by the variable speed blower could be more than the fixed speed. This will subtract from your energy savings.

I personally would get the variable speed furnance for the extra $300.

Very good point. I'll be sure to keep things clean and clear.

16 SEER unit it is-just committed and it will be installed tomorrow.

good, the vs will make a lot of difference in comfort.

best of luck.

you can find a download spread sheet calculator at
http://www.aceee.org/consumer/cooling

about 1/2 way down the page.