I find the .40 sharper recoil impulse compared to a 45acp, the 45 may recoil "harder" but it's more a push than a fast rap, just my perception.Sharper recoil, heavier larger guns (in general), less capacity in the same sized gun. I have 9mm, .40 and .45 ACP. I find the .40 a good compromise between 9mm and .45 ACP. Closer to power/penetration to the .45 ACP than the 9mm, closer to 9mm capacity than the .45 ACP. In between with recoil.
Also, I can swap back and forth between 9mm and .40 with a simple barrel change with my SIGs. Can't do that between 9mm and .45 ACP.
I've also had the major calibers along with 10mm and a couple mouse guns thrown in the mix, (not to mention revolver rounds), decided I didn't want to all those calibers to feed. NineMM and 40S&W are too easy to mix up so 40 went away, 44 won out over 10 and I've never looked back. Anymore if it's not a 22LR, 9mm, 45acp, 38/357, or 44mag I'm looking away when it comes to handguns.