It's pointless to compare them, as their styles were so different. I guess I find McCallum to be more interesting as a body puncher because he does it so well within the confines of the style of a slick boxer-puncher who fights mostly from distance and picks his shots. Not easy to blend those two things. Chavez, conversely, was more of a traditional bulldozer and very aggressive and the consummate infighter. Why would one compare them?
I remember I once asked if fighting harada had a better jab than sumbu kalambay (a hipsters delight) and the consensus was that it was kalambay because off the way he utilized the jab stylistically. It is difficult for a swarmer to have a better jab than a boxer; it is difficult for a ranged boxer to be a more effective body puncher than a guy like chavez. McCallum could break guys down though. Best uppercut to the ***** I've ever seen.
Chavez had a more consistent body attack, more effective in terms of a fight and the outcome. But in terms of shot for shot quality placement of body punches, I'm not sure anyone can claim to be unequivocally superior to Mike.