Hi snakestrikeace! Welcome to The Firing Line! I hope you find our community enjoyable and helpful. I came here after I picked up my first pistol and I've found this place a treasure trove of helpful, well-meaning input. If you're considerate and contribute to discussions, you'll not find a better community online (I post in several others and TFL is by far my favorite).
First off, you'll need to know that in every request thread, there are going to be a few people who ignore your requests. Some will criticize you for not wanting a 9x19, etc. That's fine, just scroll right on by.
Now, to address your two calibers of choice - .357 SIG and .45 ACP. Both wonderful calibers. The .357 SIG is, indeed, a necked-down .40 S&W with a 9x19 projectile that attempts to replicate the performance of the .357 Magnum. I, too, grew up shooting a Colt Python, so I'm assuming you're not recoil-averse. That being said, the Colt Python is a beautiful, heavy, firearm made of wood and steel. That tames recoil quite a bit. .357 SIG in autos has a particular snap and bark, in my opinion worse than the (also in my opinion) greatly exaggerated recoil of the .40 S&W cartridge.
A concealed carry gun will, ideally, be small and low in mass. That will exaggerate recoil. You can't fight physics, after all.. For this reason, I would like to recommend a carry gun in .45 ACP. The round is plentiful, easy to reload if you get into that, and the recoil is more of a "push" or a "thump" than a "snap", "pop", or "bang". Target re-acquisition will be slower with the .45 than most handgun rounds, but the payoff is a very powerful and very proven round.
For specific models of gun, I recommend:
Glock Model 30 or Model 36
http://www.glock.com/english/glock30.htm
http://www.glock.com/english/glock36.htm
The Glock 30 is a compact-sized Glock. They list it as a subcompact, but in reality it's closer in size to a compact-sized Glock 19 than a subcompact-sized Glock 26. The Glock 36 is a narrower .45, great for carrying, but the narrower frame and slide eliminate parts interchangeability with other Glock models. The pros here are the extreme durability and reliability of Glocks. The major con is that Glock grips tend to be a love-or-hate proposition. You'll need to feel one before you buy. That's good advice with any gun.
Springfield Armory XD(M) .45 Compact
http://www.the-m-factor.com/html/specs_11.html
This gun is a new entry into the field, but the XDM series is not. These guns have well-earned reputations for quality, reliability, fit, and finish. Customer support is also highly lauded. Well-fitted magazine extensions of varying capacity are readily available, which suits this gun (and really all XD and XDM models) very well for both home defense & nightstand duty as well as carry duty with a simple swap.
For ammunition selection, I would like to recommend any of the manufacturers and cartridges suggested at this link:
http://ammo.ar15.com/project/Self_Defense_Ammo_FAQ/index.htm
That list includes, in part:
Cor-Bon DPX All-Copper Bullet - 185gr JHP
Winchester Ranger-T 230gr JHP (RA45T)
Winchester Ranger-T 230gr JHP +P (RA45TP)
Federal Tactical 230gr JHP (LE45T1)
Federal HST 230gr +P JHP (P45HST1)
Federal HST 230gr JHP (P45HST2)
Speer Gold Dot 230 gr JHP (23966)
This being a forum for opinions, I have highlighted my favorites in bold. Feel free to retain that link for ammo selection in other calibers. Every round I carry is on one of those lists.
The .45 is not a platform I am well-versed in, preferring light-to-midsize calibers (.40 is my favorite followed by 9x19), so I'll stop my recommendations at those three models, which should serve you well.
I'm sure others on this excellent board will be able to help you more regarding guns in this specific caliber. Some will suggest compact 1911s, but I don't know enough about the reliability of micro 1911 platforms to recommend good ones.
Happy shooting!