The route over through the The Carolinas doesn't add a lot of miles to Key West, and is shorter to Savannah. However it would take you up or down what is likely the steepest interstate in the southeast, I-26 from Hendersonville, NC to the South Carolina line.
Atlanta I always go straight thru. The road surface is good. I-285 is rough, especially the west side. However I often use I-20 out to Madison GA, US-441 south to I-16. This trades the south side for the east side, lots less traffic on I-20 for the 20 extra miles.