What are you going to use it for? If you are looking for the eqivalent of a two vehicle garage, go with the two 10 X10's. The downside of this is that you are losing wall space for equipment like parts washers, welders, bead blasters, etc If you are looking at a hobby shop, go for the single 12 X 10 offset to one side. BTW, the perimeter wall is a good idea. Just insulate over the face on the exterior.