There seems to be a lot of confusion about the problems experienced with early M66's. There were, in fact, three primary issues with early M66 revolvers that were not experienced with the M19 or any other carbon-steel S&W revolver. These issues were, however, entirely endemic to the M66 no-dash variant and were resolved with the 66-1 or later no-dash variants depending on the issue.
The first issue that comes to mind is that 66 no-dash revolvers were known to spontaneously seize up after firing several rounds of .357 Magnum ammunition. This is because the stainless steel gas ring would expand more than a carbon steel one when it got hot. This issue was corrected with the introduction of the 66-1 which had its gas ring relocated from the yoke to the cylinder.
The second issue was galling of the lockwork. This arose because S&W attempted to produce an all-stainless revolver and used the same alloys for internal parts like the hammer and trigger as they did for the frame, barrel, and cylinder. This issue was corrected by switching from stainless internals to flash chromed carbon steel internals either in 66-1 or later 66 no-dash revolvers depending on who you ask. As has been noted, later stainless S&W revolvers eliminated the flash chrome and switched to the same carbon steel parts as blue and nickel revolvers most likely as a measure to simplify production and cut costs.
The final issue was that along with internal parts, S&W also made early 66 no-dash revolvers with stainless steel sights which were prone to "wash out" and become difficult to see in bright light. Later 66 no-dash revolvers went back to black sights.
As to the original question, I would buy a 3 1/2" barrel Model 27 before a 4" Model 66. The reason is because I already own both a 4" Model 28-2 and a 2 1/2" Model 66-2 and I've always thought that the 3 1/2" barrel Model 27 was the most beautiful handgun ever produced.
Smith, and Wesson, and Me. -H. Callahan
Well waddaya know, one buwwet weft! -E. Fudd
All bad precedents begin as justifiable measures. -J. Caesar