Little common sense will tell you the .45ACP is the right choice. It can be had in smaller pistol and there are many, many more defense loads for .45ACP than for 10mm auto. Companies like Winchester even load premium hollow point with reduced flash which is what smart person would choose in the first place. Average person posting here doesn't really need to carry handgun for defense let alone one with 15 round magazine. I recommend S&W Shield .45ACP with disclaimer "for very experienced handgun shooters only".
I like the size of the 20/21, longer barrel is advantageous, grip fills my hand and easy to shoot.
Need? LOL.
I've been carrying over 25 years, never "needed" it, despite that my carry has increased from a subcompact 26/27 to a 19/23 to a full size.
Capacity: better to have it and not need it than need it and not have it.
My wife has a Shield 9mm, not my idea of a primary and it won't fit in my pocket and allow for a quick draw.
Single stack 9mm (Glock 43/Kahr PM9) that fits in pocket is useful, offers a 2nd option* to the "big" pistol I'm carrying concealed IWB.
*Option to put my hand on it without revealing I'm carrying, which I can't do with the Glock 20. (Handy / comforting in certain situations)
"Need". Made my morning.