If you have three contractors giving you three different size needs this is cause for concern. Ask them how they came to the size conclusions. Did they use "manual J" figures or did they give it an educated guess?
Carrier and Bryant are both top shelf equipment. As long as premium equipment is used the one thing that will make your system function properly is the installation job including sizing of appliance and ductwork.
I would think that if the ductwork is very old say from a gravity furnace, that it would need replaced. But maybe not, I can't see your ducts or your installation so I can't comment.
Go with the contractor that is willing to explain to you how he came to the conclusions about sizing and duct requirements. The best equipment that is sized and installed poorly will never work the way it is intended and can waste your money for the life of the unit.
There are also other top brands such as Trane, Rheem, American Standard, Lennox and others. Get as high an effeciency furnace that they offer and nothing less than a 12 SEER AC unit.
Go with the contractor that communicates with you the best, as long as you are sure that they are all reputable contractors.