I have been through a number of revolvers, and think I have a stable set:
S&W Model 19-3 (Technically my wife's gun)- 4"- stays in her safe, she doesn't carry.
S&W Model 65-3- 3"- mostly a car or bedside gun, but I carry it sometimes. It is definitely my most versitile revolver, and much easier to carry than the 4" model. Yes, the extra inch does make a difference, at least to me.
S&W Model 640- 2 1/2"- this is my deep CC gun, the smallest gun in my collection, and works well IWB or OWB. It's full stainless, so not an easy pocket carry gun, but for quick trips out or in a jacket pocket, it does ok.
I've tried the 642 a few times, and no matter what, I find the extra weight of the 640 to make the gun much more shootable and accurate (for me) compared to the airweights. I've also had a few 6" Ruger Security Sixes, but the barrel just feels too long, unless I wanted to take up hunting (which isn't high on my priority list, at the moment).
If something happened, and I had to only keep one, I would keep the 3". I find that the 3" is a perfect compromise between size, shootability, and ease of carry. It feels balanced, points easily, and (as has been mentioned before) is the smallest S&W revolver with a full size ejector.
My advice, if you see a 3" revolver at your LGS- pick it up and try it, I don't think you'll be disappointed- I know I wasn't.