We are replacing two 25 year old AC systems. Each system contols a floor of the house.

We primarily live in one end of the house (on both floors) so we want to zone both floors (this is already done on the lower floor).

I have two bids:
Goodman GSX30 A/condenser with matching 2.5 ton air handler - 13 SEER
He quoted $500 per SEER per unit to raise the SEER (Thus $3,000 to get to 16 SEER). Base price is $8,995. Total for 16 SEER would be $11,995. At this point I am assuming that no new thermostats are included.

The other bid is for two Carrier FE4ANB003 Infinity Variable speed air handlers with Carrier 24ANB736 Infinity 2 speed 16 SEER condensers. I haven't asked, but I am assuming that this includes all new thermostats. Total price after Carrier rebate is $14,864. (This contractor also quoted single speed systems but the carrier rebate makes the Infinity a no-brainer. Although he sized the main floor - which is larger than the second floor as needing a 3 ton system.)

We have propane heat so this is only AC.
Just to confuse matters the second contractor is also going to provide HP bids. But that's another whole issue.

Obviously, anything we do will improve what we currently have but after lots of reading I am no closer to a firm decision. Your comments would be much appreciated.

Carrier is the best when it comes to zoning. Is this a Carrier authorized dealer? You want to be confident you have a competent contractor who has experience with zoning.

You should investigate the heat pump option. Heating with propane is expensive and you have the potential of saving on fuel costs with a dual fuel system.