I expect to purchase my first handgun soon so maybe I'm not qualified to answer, but I'll share my thought process so far -- and if that inspires anyone to give me advice also, I'm all ears.
1. I want a revolver because they are more straightforwardly mechanical and easy to understand, AND I will only have to do ONE thing to shoot it, i.e. no slide to worry about racking.
2. I tried holding different ones at a gun store and I found the S&W fit my hand better than the Ruger, and it felt somehow more "solid" to me than the Taurus. YMMV.
3. I am leaning to the 686P because I like the idea of having 7 shots rather than only 6.
4. All the gun owners I know have multiple guns but I am on a limited budget so I am trying to settle on one that will meet my needs now and hopefully also in the future.
5. The 686 shoots both .357 magnum and .38 spl. At the class I attended, which was the first time I ever shot *anything*, I was almost as accurate with a .38 revolver as with a .22 -- my shots just went a tiny bit further left than I was aiming, which I since found out is typically from not getting enough finger on the trigger... completely logical in that they didn't tell us that with a revolver you have to press with the first joint, not the pad at the tip. So I feel I can already be competent shooting .38. However, looking at a chart of stopping power of various calibers that I found, I can see that a .357 magnum pretty much beats everything else... so I'm hoping that with time, training and practice I would be able to shoot .357 magnum with it. We did not try that in the class, but I was ok although not great with .45.
6. Here in Los Angeles County, currently nobody is granted a concealed carry permit, so this is going to be a home defense weapon, with concealability less of a concern... however I plan to spend some range time and have a private lesson and try the 6" and 4" -- if I could be equally accurate with the 4" I would go with that because if we do get concealed carry here that would obviously be a lot easier.