I went through the same delima. I wanted something for IWB and occaisional pocket carry. 3" all the way.
Interesting note; I compared two lightweight guns when searching, a 642 and a 642 carry comp which is a 642 with a longer barrel to accommodate a port, I thought the longer barrel carried better in the pocket because it "stood up" more and was easier to draw due to the grip being higher in the pocket.
In my opinion S&W needs to make more 3" J frames. A #" 640 in magnum would be awesome. BTW, I was very impressed with the SP101.