Louis wasn't weak. watch his fight with Carnera and watch Louis spin the 250 lb musclehead in the clinches before ko'ing the Alp. louis had all the strength he needed in the ring.
Two common misconceptions about Louis are that he lacked footwork and physical strength. It is more the case that he tried to be economical and avoid expending energy uless he had to. The younger Louis did use footwork and manhandle opponents, but as he became more experienced he didn't need to.
Id take Joe Louis to win for basically the same reason Id pick Liston to flatten Foreman. Just sharper, quicker and the better boxer. Joe Louis has a bigger advantage in handspeed, combination punching and punch variety. Its possible that Sonny puts Joe Louis on the canvas at some point but even then Joe would get up pissed off. That's when the real whipass begins. Joe Louis TKO 8th in a fun entertaining slugfest.