IMHO, I'd MUCH prefer the M19 to the M66 (the stainless steel version). In a medium framed 357 I think the difference in steel is significant.
However, I prefer the stainless steel GP100 to the blued one because:
1) Ruger's bluing isn't very good, and
2) the difference in strength (in this design) is not significant.
For those who have not heard it here, there have been a NUMBER of compaints about the quality of steel in current stainless steel S&W revolvers. And since S&W no longer makes blued revolvers...