Law enforcement changed to the 40 because of the perceived benefit of a larger round. It actually started life as the 10mm auto and eventually was shortened to utilize the same short action of 9mm handguns. The change happened after the shoot out in Miami in the late 80s-early 90s. An officer shot a perp with a 38 special and the bullet was 1 inch from his heart and did not stop the perp from killing a few of his fellow officers.
When they started training with 10mm they realized it was snappy and harder to control. Eventually the 40 became standard. But powder technology has improved so much that 9mm=40=45 with today's capabilities. Does 1 or 2mm in diameter make a huge difference? "But the 45 weighs so much more!" Yes it does but it's also flying much slower.
True stopping power comes from the barrel of a shotgun or rifle. Revolvers are also very good stoppers from 357 and up. But for the time being, 9, 40, and 45 are pretty much equal. Personal preference is the main decider from there.
This post was edited on 12/8 at 1:04 pm