16 SEER A/C on older furnace
True...but it's true for the 14 SEER unit too. It's a package deal. The condenser, evaporator, coils, furnace, etc. all need to be replaced to achieve the SEER rating. 

Remember, the SEER rating is the MAXIMUM that is achieved. Depending on the number of heating days verses the number of cooling days per year is important too. Typical savings usually is about $10 - $20 per month from 14 to 16 SEERs. (Even 21 SEER unit has a savings of only $50 - $75 per month.) So, if you're COOLING about 6 months a year, your savings is about $60 - $120 per year. (We don't recommend a 21 SEER rating because the unit is two stage, complicated and expensive to service.)

By the way, here the cost difference between 14 and 16 SEER is about $500 - $700. (Installation cost is the same.) 

Other considerations:
1) Here, we heat most of the time, so we are concerned about the HSPF rating. 
2) Warranty (actually, we like Trane, Lennox, Carrier equally. Also, I just heard about a recall on some Carrier units...be sure to verify this.)
3) A programable thermostat will save a lot.