I've carried them all, and the SP-101 is a little easier to tote in pocket. That's about the only difference, other than the extra bullet in the Six.
Prolly get away with that in Alaska, but down here, I tote an ultralite .38 for a pocket revolver. My SP101 was carried IWB, wish I had another. If I get another, it'll be a 3" version. There is no way my Security Six was as easy to belt carry, was heavier, but fatter inside a belt and just not as comfy. The SP101 gets my vote hands down, plus, it's a much stronger design. Over the years, my security six did shoot a little loose. Wasn't to the point of a trip to Ruger or anything, just sayin' they're not as strong as some seem to think. That thing got a steady diet of hot .357, though. It's no Blackhawk in strength. The SP101 with its better lock up system was about as strong as such guns get IMHO. My personal preference, anyway. Guns like the Security and Speed Sixes get a cult following of believers over the years, the "don't make 'em like they used to" crowd. Well, they were good guns, a step up in strength over the K frames, all that, but they are lauded out of proportion now days on boards IMHO. I know, I actually owned one for quite a while and lived with it. I actually prefer my Taurus 66 for just shooting, a lot more accurate, better trigger out of the box, far more refined, just as my M19 K frame was even though neither was as strong a design as the Six. The little SP101 is smaller, though, and for a carry revolver considering the brute strength of the design, in .357s, IMHO it has few equals. Add to that the trigger is smoother out of the box than the Six was and a spring kit lightens it considerably as does the Six. Also, I much prefer the grip frame design, allows for a round butt Hogue grip which just feels awesome on the SP101 and is MUCH more compact than the Hogue I put on the Six. It's a true round butt profile. My six was a square butt, but I would still prefer the SP101's grip frame over a round butt six.
Again, JMHO as one that has owned both in the past. Neither gun is a BAD choice for carry. No one's a loser, here.