Hey there Ziptie!
Buy the one that you're most comfortable with and that you'd stake your life on (if your looking for a handgun for defensive use).
I've owned .40's by many of the manufacturers you mention and I would heartily recommend the H&K USP40 Compact or Sig P229 (current manufacture)over the other 40's mentioned. Many claim that the USP is chunky, but it is really no bigger than the Glock 23. The USP takes some getting used to, but it is the most dependable .40 I've ever shot.
I've owned many Glocks, and if you were to go that route I would suggest a 17 or 19. They are the most reliable of the bunch (in my experience) and have fully supported chambers. Parts are also very universal and easy to find for Glocks. The larger caliber Glocks do not have fully supported chambers, which could be an issue if you plan to fire handloads (not that Glock recommends it). The non-fully supported chamber is considered a weak point by many, although I seriously doubt you'd have any problems if you stick to factory ammo.
The early Sig P229 .40s had some teething problems, but I've shot some current production 229s and they are absolutely terrific. Definitely something to consider. If you're buying a gun because of recent ebents I would take a hard look at caliber choice. The US and most other militaries use 9mm NATO for their sidearms. Having a pistol in that caliber almost guarantees an abundance of ammo (one reason I'm picking up a Glock 17

)
The Kahrs are great guns, but again I'd stick with the 9mm, preferably a new manufacture P9. I own a P9 and P40 and the 9 is much more reliable and holds more rounds (9 versus 7 total). The P9 is also a little slimmer. Be aware that if the P Series Kahr you're looking at does NOT have a screw onder the slide stop going through the frame, it is an older model that will need to eventually be sent in for the modification. My P9 doesn't have the screw and functions fine, but I've heard of others having reliability problems with older guns. The P9 is a joy to shoot (unlike the P40, hey whady expect form a 17oz. gun firing the .40?) and is easy to maintain.
Boiled down version (based on my experiences - with reliability and compactness in mind):
In 40, I'd buy-
1st-HK USP Compact
2nd-Sig P229
3rd-Glock 23 or 27
In 9mm
1st Glock 26
2nd Kahr P9 (new manufacture)
3rd Sig P228 or P239 (depending on ergonomic preference)
Hope this helps and remember to buy the pistol that best fits you and to fire at least 200 rounds through whatever pistol you purchase to break it in. Let us know what you decide.
Chris