The answer to the OP's original question can be gleaned from simply looking at the .38 Special revolvers currently made and the prices being asked for them.
In Ruger's line, the only .38 Special-only gun currently cataloged is the LCR. While the LCR is available in .357 Magnum, the .38 Special version is both roughly 10% lighter and, more importantly, substantially less expensive (MSRP is $90 less for the .38-only version). Of all Ruger's other DA revolvers, which all have steel frames, no .38 Special-only version is cataloged.
S&W makes a wider variety of revolvers than Ruger, but we can still explain some of the reason for .38-only revolvers. The most popular revolvers S&W makes are their J-Frame snubs and the .38-only airweights are the best sellers. While S&W does make .357 Magnum revolvers as light, or lighter than the .38 airweights, they all use scandium or titanium alloy and are much, much more expensive (up to double the price of a .38 airweight). The only steel-frame .38 J-Frame cataloged is the Model 36 which is part of the Classic Line and is marketed as much to collectors as to those looking for a defensive revolver (the older M36 was only ever offered in .38 Special).
Unlike Ruger, S&W does still offer a few of their larger steel-frame guns in .38 Special only. These include the stainless Models 64 and 67 and the blued or nickel Model 10. I suspect that the M64 and M67 are still offered to cater to Prisons, Private Security Firms, and the few small police agencies that still want their officers to carry a revolver, but are uncomfortable with the more powerful .357 Magnum cartridge. The Model 10 is, like the M36, part of the classic series and offered for the same reasons.
So, for the private individual looking for a medium-frame revolver, there really isn't much reason to buy a new one in .38 Special unless it's for the nostalgia of a classic model. If you're looking for a small-frame revolver, however, a .38 can be had substantially lighter and/or cheaper than a comparable .357. If you're looking at used guns, a .38 can often be had for much less than a comparable .357 Magnum and there are certain models and configurations available in .38 that may have never been offered in .357.