I formerly believed that I wanted a .45 because of the knock-down power, but a seasoned LEO put it to me this way: "If I shoot you in the foot with a .40, it'll hurt just as bad as a .45" and he's right, size is relative when molten metal is being passed through your body and adrenalin is pumping rampant through your veins... What's important is not caliber, its shot placement; any reasonable caliber will be effective if you hit a vital organ. So, your best bet is hit the range, wear yourself out and tweak your nerves and hit the range again with the weapon you're most comfortable with and is most reliable (Glock is the right place for this).
That being said, the .40 is slightly smaller, easier to carry, and you get more accuracy and better range with it as opposed to the .45. The G23 is slightly smaller than G22 and same caliber. To piggy-back on what Smokey said, it depends on your use, if you're carrying on a duty belt, I'd carry the G22. If under a sport coat, G23 and if concealed go with G27. I think .40 is the ideal caliber though.
YMMV