I'm a huge 40 S&W fan. It basically splits the difference between the 9mm and 45 ACP. I have two Glocks, a G22 and a G27. I carry my G22 when concealment isn't an issue. In the warmer weather I carry the G27. I shoot the G27 as well as I shoot the G22 so I'm not at any disadvantage when I carry the smaller pistol. Size wise the G19/G23/G32 is right in between the two Glocks I have.
I think my next Glock is going to be a G19. I want to get a 9mm mostly for plinking. I don't know if I will carry it or not. The next pistol I buy after the G19 is probably going to be a Smith & Wesson SW1911 45 ACP. I don't think I'll carry this one. Personally, I would rather carry a 9mm Glock then any 1911, thats just me. There are a lot of people that won't carry anything but a 1911, and rightly so, everyone has their preferences!
As far as the 9mm -vs- 40 S&W go's, its up to you. I would recommend the 40 S&W. However, if you shoot a 9mm better then you shoot a 40 S&W, you'd be better off going with the 9mm. You can have the most powerful gun in the world, if you can't hit what you want with it......it's worthless!