you have carried a 9mm but now you want to carry a 45. I think it is odd that you started with a 9mm. In the field, Iraq, they have seen where it takes 11 rounds from the M9 to bring down one enemy. I like to do things right the first time, that means find a caliber that brings them down with one shot. That is the .45 ACP! Kimber is priced to not sell. Sig is a great gun and is easy to get back on target. Springfield Armory XD gives a nice price, but it has the polymer frame. Polymer frame gives a rotating recoil, this is the same on the Kahr. Best bet is to find a place that will let you try out the different versions of the 45. I like the 1911. My use is person safety in public and family safety while in bear country. The trouble with bear is they do not fall on the first shot.
I can promise you that there is no such thing as the one shot stop. Period. Do the research and learn.
COM hits (Center of Mass) are also a huge falacy and what you want to be aiming for is the center of the circulatory engine, which is a triangle roughly running between the nipples with the apex at the base of the neck. Stick a pie plate in the middle of that and you are going to get a 'bleed out' which will leave your bad guy with his engine running even with a hit to the heart itself, believe it or not.
The second triangle that one wants to consider is the "one shot stop" triangle. Its base runs along the lips horizontally with its apex at the bridge of the nose or base of the eyebrows. A hit in this zone MAY be a one shot stop but no guarantees although chances are much better as you are going to do mucho damage to the cerebral cortex.
In terms of calibre, the jury is still out after several years of study. The answer being given is to shoot what you can control, starting at 9mm. and moving upwards from there. There are many other considerations as you can well imagine.
My suggestion to you is to use and shoot regularly a 9mm and a .45 that mimic one another. I have both pairs (i.e. a Glock 26 and a Glock 30, both with NY1 triggers). I carry the G26 when it's warm out and I am wearing lighter clothing and I carry the G30 when it is cooler and I can use a heavier holster/belt/cover. Either way I do not feel undergunned - it's six of one and half a dozen of the other. The 9mm allows me very accurate shot placement while the .45 does more serious damage in most cases. I carry what will take +P+ rounds as I want maximum energy imparted to what I hit and maximum expansion and shock. Looking at forensic evidence of both types of rounds one will note that the damage done by either, in a well engineered bullet, is similar, with the .45 giving a slight advantage in a bleed out situation.
The essence that you should be remembering is that both guns, if you choose to carry two different ones, should have similar controls; you should use the best rounds money can buy and you should train to shoot fast enough to get away lots of rounds and slow enough to make those shots accurate. And you need to stop shooting when the threat is stopped, otherwise it is murder (in more ways than one).
Lately, I have started playing with a Sig P239 Tactical in 9mm and believe that I just may start carrying ONE concealed carry and that will be it. If I can place accurately two or three rounds of 9mm versus an accidental hit with a .45 I will go with the former. If I am faced, however, with having to shoot through obstructions like glass or drywall on a regular basis - I think I would probably go with the .357 Sig (or failing that with the .45 +P+).
There is no BEST .45 ACP really. It is whatever you are most comfortable with IMHO. It is what conceals well. And above all, it is what is TOTALLY reliable. Unfortunately, for me, the list of 1911's that I would bet my life on is a short one. Very short.
I can assure you that 5,000 rounds put through a Glock .45 of ANY kind is going to come out with close to zero malfunctions, whereas put through a 1911 you are going to get a whole handful with most. That's just the way it is and that's why I am using 1911's for competition where my life does not depend on it and a striker fired handgun like a Sig where my life depends on a dependable accurate shot. Or a Glock. I love the Glock 30 and it seems to me that I have heard experts like Ayoob say that it is a favourite carry (along with his P345). The choice is, of course, not an easy one and for sure it is not black and white.
So I muddle along with my Glock 26 and Glock 30, hoping that one day I will have conclusive evidence that well placed 9mm engineered rounds moving fast are going to get me the same or better results than .45ACP rounds that are not as well placed.
And then there's the controversy between wheel guns and semi-auto's. Another discussion?