Accuracy should be eqivilent. The stainless barrels are very good accuracy-wise and that is the only difference between the models: 67=stainless; 15=blued moly steel.
Have one of each. Both are more accurate than I can hold most days. Off sandbags they also seem to sooth the same. The 15 is from the early 70's and the 67 is a 2005 production version. Ditto the results between the model 10 and 64. S&W makes superb .38's.
I have in my collection most models of S&W revolvers. I shoot the M15 4" bbl. the best of all. It just feels perfect in my hands with it's original Diamond Magna Grips and S&W grip adapter ( similar to the Tyler-T. ) A perfect example of a classic S&W revolver.......
My belief, and it may be wrong, is that carbon steel guns are better, on average, than stainless. I'm sure that I could never prove that, and wouldn't even bother trying (or worry about it), but it's just something I've always believed.