Do you know that Carrier and Bryant are the same equipment ?
Why don't you get bids on diferent brands of equipment.
There is no reason to stick with a Carrier or a Trane, in fact these units are probably very low on the list of quality but high on name recognition.
Choose the contractor you trust and don't get hung-up on the difference of a few dollars geting a bad install won't be worth the difference.
Don't look for someone to do a load calc, if someone came to my house and told me he needed to do a load-calc to choose the proper size of my equipment I would show him the door ( on the grounds that he probably doesn;t have enough experience) Contractors use load calcs as a tool to try to build the customers confidence, other than that they are worthless.When a customer asks me to a load calc I agree then tell him that there is a two labor charge for the service.