In my experience with 1 7/8" 2 1/8" and 3" .38s and .357s the extra barrel length makes a lot more difference than you'd think.
With a fairly light target load (3.2 gr Red Dot, 158 gr LSWC), I'll average 670 fps out of a 1 7/8" Model 36, and just 675 fps out of a 2 1/8" Model 60, but I'll get 727 fps on average out of a 3" barrel.
That compares to an average of 748 fps to in a 4" Model 10 and 794 fps in a 6" Model 19.
So with a light load using a fairly fast burning powder, the 3" picks up about 50-60 fps more than a snub nose, and loses only about 20 fps to a 4" and about 60 fps compared to a 6" barrel.
With a +P self defense load (7.5 gr 800x and a 125 gr JHP), the 1 7/8" model 36 will produce 929 fps second on average, with the 2 1/8" producing an average of 951 fps - not much of a difference.
The 3" barrel in contrast gains nearly 80 fps with an average velocity of 1030 fps. This compares to 1100 fps in a 4" Model 10 and 1186 fps in a 6" Model 19.
The heavier load and slower burning powder reward longer barrels, but the gains in that extra 7/8" to 1 1/8" of barrel are still significant.