The only problem with loading the 40 "Nuclear" (beyond SAAMI Spec) is that Glock built the 40 on a 9mm frame and that platform is already maxed out as it is.I went through this very same thing a few years ago. I own both and am confident that either gun will do whatever I need as far as personal defense. Here's what I found in my research: All things being equal, the 29 will give you more power than the 23. I chonographed 165 Gr. bonded loads from DT in both calibers from my personal guns. G23 was averaging about 1175FPS G29 was averaging about 1350FPS
My handloads worked up to SAAMI max (but not above) gave me the following:
G23 180 Gr Gold Dots averaged 1200FPS.
G29 180 Gr Gold Dots averaged 1385FPS. The muzzle flip was pretty heavy but controllable.
G23 165 Gr Gold Dots averaged 1230FPS.
Is it enough to make a difference in the real world? That's up to you. For me, it does make a difference and I chose the 29.
If you decide to reload nuclear, the 40S&W is a formidable round and will do whatever you need. But I don't reload defensive ammo, so again, I went with the 29.
So it makes a lot more sense if you want to go "Nuclear" to do it with the 10mm instead of the 40.
Trying to stuff the most powerful cartridge in the smallest possible gun only results in less controllability, poorer accuracy, and slower follow-up shots.