But, really, get the highest effiency rated and best warrented equipment you can afford.
If your system is 20 years old you will be amazed at the $$$ you will save on utility bills. You will also be amazed at what the new system will cost. Just remember it will last a long time and some have lifetime warrenties.
DON'T GO WITH A COMFORTMAKER!!!!!! My house is 4 years-old and the contractor put that model in and it's been a piece of junk the entire time. It may have been the technicians who installed it, I don't know. Freon leaks, fan coming off, motherboard and temp switches for the defroster cycle, etc. If you plan on living in the house for a long time, go with the good-name brands. JMHO If you still go with it, think about the annual service plan.