S&W was continually searching for ways to simplify production and reduce costs,
The Model 14 and Model 15 had different barrels and frames until the older tapered barrel 15-4 design was discontinued,

After the 14-5 got a full underlug barrel in 4", 6" (and 8 3/8" ?) barrel lengths and the old version of the 14-4 became the new 15-5 available in 4", 6" and 8 3/8" barrel lengths (bizarre right ?)

OK so that being said the 6" 15-5 is basicly the same as the (non pinned barrel) 14-4 with a new model designation,
While they are not as commonly encountered as older variations IMO they warrant no premium.

The shorter 4" version is interesting to 4" collectors as it mimic's the 4" Model 14-3 but with a non pinned barrel and cheaper finish.