As for Sears, they contract out to a 3rd party, thus, there's a FEE for getting the 3rd party your job.
Aside from that, what do the contracts say regarding the work? Does 1 contract say the rotted siding is to be removed (and disposed of), and the 2nd contract say the rotted siding will be sided over? Also, find out the make/model of siding being used. Then, find out the price differences. Contact the mfgrs. as to where you can buy it, then call them for pricing. You may find quality differences. You might want to find out if the Installers are Bonded and Insured. These cost $$$ too. (A bond may not be 'applicable' in this case, technically speaking, but it's worth asking.)
Other than that, I'm out of thoughts. My best to ya and hope this helps.