Don't leave home without it.
Go with a .45 ACP. I've been a police firearms instructor for 25 years, and I know of more than one occassion where a 9 mm did not do what it was intended to do. Just my .02
Hey, Gimme my hat!
I'd go for a 9 Largo (9mm Bergmann-Bayard), loaded with THV (the fourth bullet in the image, here fitted to a .38).
The 9 largo case, being cilyndrical, doesn't have the nose-dropping feeding problems associated with the 9 Para (conical frustum) in large magazines, so you can load a 25 mag with no problems.
With a spare mag, you're packing 51 'lil friends'. Enough to win a little war...
Just my 0.02€
In the end, the choice has to do with if you are going to carry it regularly.
Some people find that a large & heavy gun becomes a burden for conceaaled carry.
Smaller guns can then be a handful when trying to shoot.
Some lightweight guns can be punishing with recoil.
As said before: while 45 represents the better round for 1 shot stops, it is the gun that you have with you when needed that is the best.
I never rely on a single round stopping an attacker no matter what the caliber, that's why I always load more than one.
My 10mm auto packs a lot more more punch in the same 1911 platform but is not as easy to handle, and we can go outside the 1911 platform and go revolvers. The .357 mag, .44 mag, and the old .45 long Colt are much better stoppers than the .45ACP any day, but in a similar sized carry package do not handle nearly as nicely as the .45ACP. For most of us the .45ACP is the best balance of stopping power, terminal ballistics effectiveness vs recoil comfort. I can handle the other cartridges but under controlled conditions. In a rapid fire situation they tend to get out of hand as for as controllability fairly quickly. For defense I want something I can controll quickly in a follow up shot situation, and for me it boils down to the .38 Special, 9mm, or .45ACP!