When faced with this decision, what key points should you consider?? I would suggest Operating Costs, repair costs, and estimated life expectancy.
OPERATING COSTS: Old versus New! Example: a new furnace at 95% efficient indicates that for every $1.00 of energy used to operate the furnace only five cents is not fully utilized. If you spend $100.00 to cool your home with a 10 SEER unit upgrading to a 16.0 SEER unit should cost approximately $68.00.
REPAIR COSTS: Example: replacing a condenser fan motor would cost $300.00-450.00 depending on the brand of unit. A new unit purchased from Controlled Comfort would include a 12 year parts and labor warranty.
LIFE EXPECTANCY: The main component of a furnace is the heat exchanger-life expectancy is 20 years. The main component of an air conditioner is the compressor-life expectancy is 10 years. If you buy a new system from Controlled Comfort it would include a lifetime warranty on both components to the original owner.
To sum this all up, if your system has seen better days, upgrading may be the answer. The end result is to save dollars out of your pocket and save energy.