Given your location in South Carolina, I would start with a 30-F bag like the Megalite. That's the bag I use from March through mid-November in the Southern Appalachians. It's warm enough to take well below freezing (if you have the right clothing), but not so warm that I need a separate summer bag. It is comfortable in a wide range of conditions.

For winter use, I have a WM Antelope 5-F bag. If the nighttime temps are above 25 or so, I roast in this bag.

My personal opinion is this: the "conventional wisdom" of the 15 or 20-F bag being the all-purpose holy grail is not correct for where we live. It may be the perfect 3-season/summer bag in the Rockies, or in New Hampshire, but it's both too warm and too cold at the same time around here -- too warm from April through Sept, and too cold in the winter. It's probably perfect in March and October, though.

Hope this helps.

--Ken