Bart, the more I've learned about ballistics, the more I've learned that ballistic gelatin is simply a medium to compare ammunition. I think a much better indicator for performance would be any sort of barrier either in front of, or within the ballistic gelatin.
If the .38 penetrates, and expands the same as the .357 after exiting through the barrier, then I don't see why the .357 would out-perform the .38. My gut feeling is that the .38 would not be able to perform the same feat as the .357.
The conversation sort of twists itself into the fact that there are two ways for a bullet to do it's business. Slow and heavy, or fast and light... Then you get into the heavy and fast calibers, but those tend to be only good for recoil junkies.
To further back up my point as far as barrier penetration (which includes bones in my book!) the ol' .38 158g loads were a success; the .45 230g have always been a success; then the popular .357 in 125g, of course.... The prominent "man-stoppers" have always been the penetrators; and I don't mean through ballistics, I mean through barriers.
The fact is, all major calibers are great defensive choices. The great improvement over the years has been bullet design. Ammunition manufacturers now can design a bullet to mushroom, but still not fragment, and also penetrate to whatever depth they are aiming for.
At the end of the day, it all goes back to having a reliable firearm that you can shoot accurately. Tactics overcome weapon choice 9/10.