I've never seen an RB 15...
If you cover the grip frame with a decent set of grips, like Rogers, the grip frame itself may not matter. The SB "stocks" on the 15 are kind of stocky, and certainly larger than the RB. Furthermore, I make no claim as to the relative concealability of the two, since I am not stupid enough to compare them (the factory SB "stocks" don't work very well for me, and it would be stupid for me to use them when the excellent Rogers grips are available). I have actually carried a 15 SB with Rogers grips and a RB 12 with S&W "stocks," and I can tell you that the Rogers-15 combo is just as concealable as the 12, maybe more so. I think that the Rogers may be thinner; I'm not sure. I have no complaint against either, with what I wear. It may have to do with the relationship among shoulder width, waist size and round butt; I do not know exactly.I've never seen an RB 15.
Apparently concealability is subjective. I always found a smaller gun to be easier to hide. Others, it seems, do not. I am surprised at this. Seems like simple physics, to me. I don't get their reasoning, but there it is just the same.
Is a round butt K frame revolver smaller in the grip area than a suare butt K? Clearly, yes, in absolute terms.
I would say that the degree of size difference is so minor, however, in practical terms, that if one can effectively conceal the round butt gun, the square butt gun would be pretty much equally concealed, given the same type of stocks. If an observer would spot one, he would spot the other. The cylinder of a revolver has always been the hardest handgun bulge to hide effectively.
So, is there a difference? Yes. Does that difference translate to much? No, not really.
The 2" M15 is a neat gun but I never could figure out a use for it.
Like I said in the first post, the square butt and adjustable sights work against easy concealment and carry. A round butt M10 with its fixed sights is much easier to carry and draw from concealment IMO.