All I know about the car business is that I buy way too many of them.
As a result I have a very good relationship with both Baker and Hendrick who are the two major players in Charleston. I'm not familiar with the dealerships in MB, but I can't imagine they have the number of dealerships that allow for the kind of upward mobility within a company that Baker and especially Hendrick can provide. That said bigger dealership groups tend to be a lot more rigid in their corporate practices which I would assume makes them less fun to work for, but tbh I know nothing of the inner workings of the business. Then there are a myriad of smaller dealerships in and around Charleston.
It sounds that you are qualified to work at any dealership provided they have an opening, so I would approach your decision more from where you'd like to live, explore the job opportunities there first, and then do your calculations of income relative to cost of living. And then once you do all that move to Charleston because MB is all kinds of awful.