Hi Ed, (and everybody else). # 2 or 3 on my obsession list (right after duck hunting) is pistol shooting. I've got quite a few and shoot them often and carry daily. If you can afford it, buy a small pistol that easy to hide in warm weather and the mid size gun you really want for non warm weather times. Invest in more than one holster. If you carry in the pocket, PLEASE use a pocket holster. The world is full of accidental discharges due to unholstered pistols getting tangled up with other crap in pockets.
Whatever caliber you get, buy some snap caps that you can use to dry fire practice. Its way cheaper than buying ammo and greatly aides the learning curve. Paint them, mark them, color them, whatever just so you won't confuse live ammo with them.
As to what gun? Great strides have been made in defensive ammo over the last couple of years. Where once I would have answered that only guns .40 caliber or bigger are worth considering, the truth is, that there are good loads in .380, .32 mag, .22 mag and .38 special. That opens up possiblities in smaller, easy to hide guns. Smith J frames, Ruger LCR's, North American Arms .22 mag, Kimber Solo, the Smith & Wesson Shield, and the Kahr that SJ mentioned is right at the top of the list of the next pistol I might buy. Walther PPK is a little bigger than the newest generation of ultra small pistols and is all steel, but it makes it more controllable and less punishing to practice with. Any of the new Ruger semi's would work out nice size wise.
I have to take exception with buying a lightweight .357 as the blast and recoil is very unpleasant and could lead to a flinch in an inexpereinced shooter.
I've shot the small Glocks in .40 and was favorably impressed with their shootablity and my ability to hit something with them. I have to guess that the 9mm, with its smaller recoil pulse would be even easier to master. I've never shot the Sigma or the Taurus you mention, but I have shot a couple of different Sig Sauer's. Well made guns that fit the hand pretty well. If possible, buy a pistol that has interchangable back straps and try all 3 for fit and controllability.
Good luck on your hunt...
John Bourbon