I hear Air Atlanta treats its pilots almost as poorly as Ryanair and pays absolute crapp... I'd go for Thomsonfly and then ultimately transfer over to Britannia on the 757/767 after the required years on the 737. Go for the parent company and the long-term potential. Air Atlanta is a lowest-cost operator and the pilots will always be subjected to cost pressures as competition increases in the future.
I say choose Thomsonfly/Britannia.