zoning system or 20seer system

I have 2 AC contractor come to my house.the first one tell me replace the 4 ton American standard system 20 seer condenser and 95% furnace with the cost realy hight...The second contractor give me a very good deal with the system 3 ton American standard,15seer,80% furnace and 3 zone system(my house size:1700sqf)
Can anyone give me an idea which system is the best to go.thanks

Ask for more independent contractors bids and discard lowest and highest bid.
Than choose one that has best references. And, I suggest that you check that references. If you are lazy, than hire some consulting firm.