Strictly for ammo capacity. My typical loadout is the gun in "Condition 1" and two spares in a carrier at 7 o'clock. With the S&W 45 Shield and 7 round mags that is 22 rounds of Hornady Critical Defense 185 gr .45 ACP. Since it is still winter and I can easily conceal it, I have gone back to the Ruger SR9c with the 10 round mags and factory fingertip extension. Carrying the same way and loadout gives me 31 rounds of Hornady Critical Defense 115 gr 9mm ammo. Comparing ammo capacity between the two is almost one more full mag. I could carry my full sized Glock 21 in .45ACP, but that fucker with 2 spare 10 round mags gets heavy when carried all day. I will strap that on for certain circumstances, though. Think classic "soft target" scenarios. When warmer weather returns, I will want the slimmer profile of the 45 Shield for under my biker vest, or in my pocket.
The reason for the change? The world is inching closer to WWIII. Having the U.S. protected from invasion by another country by two oceans is 18th century thinking. We are presently being invaded on our southern border by tens of thousands of people with no intention of becoming loyal legal U.S. citizens. We are susceptible to cyber attack, ICBM's whether nuclear destruction or EMP, and the whims of world oil producers since we shut down our own energy independence. Then there is the nationwide crime wave and criminals acting with impunity. Any of these factors could trigger bad actors already on our own soil to become even more emboldened.
I don't get into the whole "9mm vs .45 ACP" debate. I use both, but just like .45 a little better... its personal and a little nostalgic, thats all. Those Fudds & Butters that personally try to goad me into such discussions are still thinking all 9mm ammo is the FMJ version the Germans used in WWII. It certainly is not. I end such arguments by asking if 9mm is so harmless and ineffective, can I shoot them with it?