For a REALLY long barrel to get you a benefit, you need a lot of slow-burning powder--more than even a magnum pistol cartridge will give you; you need a bottlenecked rifle cartridge (or MAYBE the stretched-out .357 Max?). I do not have, nor have I seen, concrete comparative data between an 18.5 and 26 inch (or whatever) barrel for the .357. What I do know, however, is this: out of my 18.5" Marlin, a max published load (from hodgdon) of Lil' gun will push a 158-grain Hornady XTPFP at an honest 2,000 fps. The only load I've seen that'll go faster than that is the commercial load noted above--and they, too, clock it from an 18.5 or 20" barrel, if I recall.
If there were significant gains to be got above that 2,000-2,100 fps range by adding another half-foot of barrel, I think I'd have heard about it, and I haven't. So, if it were me choosing, I wouldn't choose the longer barrel unless I really liked it for some other reason. To me, though, the little carbine format is just right for maximizing the .357 mag's potential.