When I puchased my used G23, it had one of the NY triggers (the gray one). My groups were lousy compared to my other two G22s. I replaced the NY spring with a factory spring and my groups improved dramatically.
Regarding .40S&W vs. 9mm, I went with the G23 because I already had the G22s. If I were starting from zero, I would opt for the 9mm due to cheaper ammo. It all about shot placement, right?