What advantages does a .38spl gun have versus a .357?
Well, probably the most undebatable is price. Because of the added versatility of the .357mag round, guns chambered in it command a bit of a premium compared to .38s.
There are a few other things one could point out, though neither have to be major issues:
1. Cleaning the chambers: Shooting shorter .38spls in a .357mag will leave deposits in the chamber that, when built up sufficiently, can make it hard to seat the longer .357magnum round. Since most shoot mostly .38s through their .357 anyway, frequent cleaning of the cylinder becomes an issue. A hassle for some. A moot point with a .38-only revolver.
2. The Model 15 in that link is a k-frame. S&W also made k-frame .357s and are very popular. However, shooting a lot of .357mag rounds can add a lot of wear and tear on the gun. If you're buying a used .357, it's something to be aware of. Conversely, excessive wear and tear on a used .38 is often less of an issue.
Some believe a .357mag is less accurate shooting .38spls than a .38spl shooting the same round due to the extra distance the bullet has to travel before it enters the cylinder throat. Most agree, however, that if the effect is real, it's so small as to be essentially insignificant.
In the end, you could certainly buy a .357. Given your parameters, though, and that 1) the Model 15 is a fine revolver and offered at a good price, 2) the .38spl is a fine SD round (despite what you might read on the internet) and 3) you'll likely shoot mostly .38spl through it anyway, you should at least be aware of the J&G option.