I have 4 revolvers in .357 Mag that I shoot regularly, and one semi-auto in .357 Mag. I shoot a lot of both .357 Mag and .38 Special in all of them.
I notice a tiny little bit of sludge when I push in a .357 after a bunch of .38, but it's just that FIRST chambering AFTER the run of .38 where I can feel it -- at all. And one cylinderful of ammo and it's ancient history.
I've heard and read in forums for Y-E-A-R-S about the concern over shooting a lot of .38 in a .357 and having to deal with the side effects of the shorter round. I just have NOT had experiences that has made that complaint valid.
I'm sure everyone has their own experiences, but it's always been much ado about absolutely nothing in my 25 years of handgunning. I do tend to run plated and jacketed rather than lead in both .38 and .357 and I'm quite sure that's part of the equation, and you'd likely find more resistance with lead residue.
In the Coonan, I'll run a couple HUNDRED rounds of .38 with the included lighter 10-lb spring and then switch to full bore .357 Mag and I -CAN- feel a slight hitch when I go to chamber that first round, but it burns or blows out any of the scuzz and just like the Coonan always does, it eats through all the ammo I feed it without failures of any sort.
Personally, I file this "concern" under "duly noted", where I always keep it in mind as something to look for, but haven't ever been bothered by it, and I'm talking easily more than 10,000 rounds of ammo over a couple decades -- and the last 5 years of that has been documented with hard, real numbers.
As to the accuracy difference -- the only thing I can seem to tell in difference is that point of aim can be different due to the radical difference in recoil. I do love very much shooting both, and I shoot a lot of .357 Magnum. It's not that I'm a tough guy who likes taking a beating...it's more that my guns tend to be on the larger and heavier side and they eat up a lot of the recoil. The Coonan -LOVES- to be pushed hard and demands it from the ammo.
Attention Brass rats and other reloaders: I really need .327 Federal Magnum brass, no lot size too small. Tell me what caliber you need and I'll see what I have to swap. PM me and we'll discuss.