"Stopping Power" is a way more complicated topic than it seems, as witness the huge amount of bandwidth it has sucked up over the years in GT's own Caliber Corner. Because life and death are involved, some folks treat it as something close to a religion, and even become "crusaders" for their "ballistic belief system."
Personally, I think each side holds a piece of this very complicated puzzle. I am partial to the collected street results, and keep in contact with police firearms trainers around the country through organizations such as ILEETA to see what's working best "on the street." The collective feedback from there tends to validate Marshall's and Sanow's recommendations. However, both Marshall and Sanow also use ballistic gelatin in their testing and evaluation.
Dr. Gary Roberts, perhaps the leading proponent on the other side of the debate, obviously relies heavily on FBI/IWBA protocol gelatin testing, but also makes a point of gathering feedback from police and military users. So, both "sides of the debate" obviously see value in both lab testing and monitoring street results.
If doubt about which theory is right creates a crisis of confidence, you can just buy ammunition that both sides hold in high regard. That includes the 158 grain lead semiwadcutter hollow point +P in .38 Special for example, the 230 grain jacketed hollow point in .45 ACP, and the 180 grain subsonic JHP in .40 S&W. Both factions seem to feel that modern high-tech bullets have performance advantages over conventional "old school" JHPs.