I'd vote Alaskan slab as well. You won't have animals moving in below building like you would with the other design. Also I doubt that you need 24x36" at the perimeter. The IRC has prescriptive Alaskan (FPSF) foundation requirements now. You can find it online.

Also, are you saying you're putting 2 interior posts under the 6x8? You could get rid of the posts if you beef up the beam. It's only 16', and you've got some nice hardwood in NC.