There is no such thing as "knock down power". In order for there to be enough power to physically knock down the person being shot, it would have more power in reverse (because of loss reaching the target) and knock down the person shooting the gun.
Aside from that misunderstanding, the 45acp is a better caliber because it's a larger hole, thus causing more blood loss and the increased possibility of hitting a vital spot. Along with it's greater mass which will have better penetration. Not necessarily more, but better. And the 357 magnum, head to head, with the same weight bullet, will usually be a more powerful cartridge because of the revolver and the hotter loading. And anything you load the 40sw to, you can load the 357 magnum hotter. The 40 has a slight diameter advantage over the 357, but not significant considering the bullet weights are the same and the 357 is loaded hotter.
The 40 is better for people who aren't comfortable hitting their target with 6-8 rounds. If you are comfortable shooting a revolver and 6 rounds, or a semi-auto with 7-8 rounds, then the 357 magnum and 45acp are better calibers. If you aren't comfortable hitting your target, need a lot more rounds to compensate, or want to audition for lethal weapon pt6, then the 40sw is a much better caliber.