I'm thinking of buying a NIB M&P Shield 9x19mm no thumb safety.
I considered the .40S&W
but the 9mm has a few +s.
It cycles/feeds well, a + for the small size/short barrel.
It recoils less & less blast/flash than the .40S&W in most loads.
It holds more rounds(1 or 2) than the .40S&W in the magazines. That's not a deal breaker but it's a good point.
A 9x19mm might be slightly better to shoot while wounded/hurt or if needed, one-handed.
9mm JHPs & defense loads aren't hard to find & milspec rounds aren't super expensive. The .40 is not bad either in most areas of the USA but Id prefer the 9x19mm.
To my knowledge, no after market sources produce a .357sig Shield barrel.
Id heard S&W
used to sell 9mm barrels for the Shield .40S&W but they cut the product.
Overall, a 9mm Shield would be great for any armed citizen/CCW license holder/sworn LE officer.