I'm with everyone on the .40 . Love my 10mm's, probably the best perp stopper ever made, and makes my .45 almost a toy by comparison. But I hafta handload it these days. The .40 came about from S&W developing their 10mm's for the FBI. FBI wanted a LOT less than what that 10mm was delivering. My PT99 Stainless Taurus is completely convertible from .22 LR to 9mm to .40 S&W and .357 SIG. Like that gun alot! (Beretta frames were made by Taurus for a number of years, so they learned a few things). RJ loved it when he was smaller in 22, then with a compensated 9mm upper, now uses it in .40 quite a bit. Don't really see what the hype is on the .357 SIG, but it is a bit more accurate at longer ranges, say 75 yards, but does not have the knockdown power of the .40 at the same range.
I don't see where the .40 is less plentiful for ammo than the 9mm? Maybe surplus ammo, but I wouldn't be running that cheap crap through anything but an old beat up Mac 10 or HiPower or something. Unless you really enjoy completely tearing down your guns to the last pin/screw and cleaning for hours......
Personally, I carry one of my .45's or 10mm's regularly.

Depending on what 9mm you buy Marc, you should be able to easily and cheaply find a .40 S&W upper for it. Then all you need is the magazines to convert back and forth, but were I doing it, I'd buy the .40 version, then shop Gunbroker or other places for the 9mm upper parts for what you buy. Far easier and plentiful to find those! And there are .22 kits available through Ceiner etc for like the Beretta and Taurus. I doubt you'll find a "Pistol Pac" like I have from Taurus, but it can be duplicated with other parts readily available out there on many guns, the Beretta and Glock included
Ray