For self defense all the calibers mentioned work well be it 9MM, 40 or .45acp.
For playing the shooting sports such as IDPA/IPSC then 9MM, 40 & .45acp all make good sense. Just depends on what divsion you want to shoot in and with what.
My understading is that the use of personal firearms for "self-defense" or "home defense" in Canada is non-existant, technically. All restricted firearms, except those carried by peace officers are for target shooting (unless you live in the bush), so the caliber as used for such reasons is moot. That being said, I'm not an idiot and my 9mm is legally stored yet easily accessible in easy reach of my bed.
LEOs started going over to the .40 cal, like the FBI, after incidents like the Miami shootout, where they weren't able to effectively take down a suspect with 9mm ammo until shooting him repeatedly, in the meantime he's shooting at them. But in that incident, and probably others, the suspect was heavily armed with rifles he could keep the police outside a more effective 9mm range with, so most of the hits weren't the effective hits one would likely receive in a personal defense or home invasion scenario. The only people armed to deal with the situation of a heavily armed bank robber is the police and if they have to be able to render more damage to him with every hit, they should be using a larger caliber, which they do. But if a guy took a 9mm hit inside a home or within 10 yards, he would most likely suffer a lethal wound immediately if the shooter is even a little skilled. I think 9mm is enough for this purpose.
Another upside is the better price and smaller kick for target shooting. I find 9mm comfortable to shoot at a high speed or with double taps, putting both rounds where I want, and notice a huge difference when shooting .40, it being just a little too big for that purpose.
As for the zombies, everybody knows you have to score a headshot to kill a zombie, so shooting it 10 times in the body with 9mm, .40 or .44 mag will do nothing but all three calibers are equally effective when used on the zombie head. That's the first thing you learn in zombie-invasion school. But in that case, you're better off having more rounds (offered in the regular 9mm magazine, not the canadianized) and smaller kick for more accurate follow up shots, or quick succession shots on a large number of zombies in a horde offered by the 9mm round.
I'm not an expert on the balistics and theory of law enforcement firearms so that part's just an opinion. But I'll be damned if I'm not an expert on Zombie invasion theory!
