hmm, i guess that depends upon whether you prefer to capitalize "the South" when talking about the region from whence this phrase came. Since "the South" is in reference to an actual place, understood as such-and-such an area in the southern US, you could treat it like a place name, and capitalize it.
so, southern is just an adjective denoting cardinal location, and Southern is an adjective denoting regional affiliation to the southeastern US.