From what I have seen and read, the Kentucky long rifle had a straighter comb and less drop of the butt, while the Southern Mountain Rifle has a more rounded comb and more drop. Also, the Southern rifles were iron mounted, since iron was available for mining and cheap compared to brass which was usually imported. It got plainer the further South one was. Note that the "Kentucky" rifle received that name from a song, and was made by various gunsmiths in Pennsylvania and the New England states. The Southern mountain rifle was an offshoot of the Pennsylvania (Kentucky) rifle and was made in the mountains of Tennessee, the Carolinas and even in Georgia.
Now (disclaimer), this is what I have come up with from my reading. I hope others will chime in and correct me if I am wrong....
~Kees~