I carry either a 2" or 4" S&W K Frame on a regular basis, and have no trouble doing so. In that regard, an SP101 would probably be even easier. I shot an SP once, and did very well with it. Awesome little gun. I do a bit better with the Ks though, so I've stuck with carrying them. The extra round is a nice bonus too, but like MC, I wouldn't feel undergunned one bit with the SP.
Maybe I'll buy a used SP101 one day. They are cool little guns. I just prefer blued finishes myself, wish Ruger would offer the SP with a blued option.
I do own a 4" Ruger Police Service Six. Comparing it to the closest sized S&W I own, a 4" Model 15, they are VERY close in size. The Ruger's cylinder is just a smidge wider (my calipers are busted, but my eye figures around .1" at most). Call it even. Interstingly, the Ruger's grip frame is noticeably smaller.
In the end, any would work great, it is just a matter of finding the best match for you.
A round butt K frame would be more compact in the grip area.
That would be another option. Both of my 15s are square butts, but I recently bought a Model 13 .357 Mag with a 3" barrel and a round butt. The round butt does make a noticeable difference in concealability and comfort when carrying IWB, though I don't have any objection to the SB models. Once I've proven it to be reliable, I'll start carrying it with either a 158 grain 38 +P, or maybe Speer Gold Dot's .357 135 grain JHP. Those Speer .357s are fairly mild by .357 standards, so the 13 ought to do fine with them.
On a final note, I find that the SP, K Frames and the Ruger Sixes all balance/point pretty naturally for me. But my hand tells me that the 3" K Frame pictured below is the sweetest balancing handgun of all (due to family issues, I've not had time to shoot it).
