I currently own one of each, but mine aren't directly comparable as the G23 is a Gen3 RTF2 model.
Either is a fine choice; really, it just boils down to what caliber you prefer. Here are the stats:





Fifteen slightly-less-powerful 9mm caliber shots, or thirteen slightly-more-powerful .40 caliber shots.
The .40 weighs more (empty or loaded), but not by much.
9mm will bounce a bit less during firing.
.40 ammo is about 25%-35% more expensive (comparable loads).

Practice being important for building skill, and cheap practice ammo being better than expensive practice ammo (more bangs per buck), I'd say unless you have a need or strong want for the .40, go with the 9mm.

Rent both if possible and shoot both and see which you prefer. You will know which is best for you when you are done. Dont get caught up in the the caliber debate that always goes on. Whatever you shoot best go with it.

I either buy 9mm handguns or .45 ACP. It's my own personal opinion that a 9 will do the job and so will a .45, I just like .45s.The .40 is just an "in-betweener", but there is nothing wrong with any of them. The 9 just has less kick/recoil than the .40. My .45s feel like a "push" rather than a sharp recoil. Just my two bits. Center mass hits will work with any of them.