The most obvious differences are as follows:
The M19 was only offered with adjustable sights while the M10 was offered with fixed sights only. The M19 was available only in .357 Magnum while the M10, with the exception of a small run prior to the M13's introduction, was available only in .38 Special. The M19's standard barrel lengths were 2 1/2", 4", and 6" while the M10 was offered at various times in 2", 3", 4", 5", and 6" lengths (small runs of other barrel lengths may have also been offered for both models). The M19 also had a longer magnum-length cylinder (longer in fact than the N-Frame M27 or M28) while the M10 for most of it's production run has had the shorter .38 Special-length cylinder (more recent examples may have longer cylinders). The M19 was made only with heavy, half-lug barrels while the M10 was available with both heavy and tapered barrels but not ejector rod shroud. Also, on 19-4 and older guns, the cylinder was recessed while the M10 has never been offered with a recessed cylinder.
Similarities between the two models are that both are K-Frames, both were carbon steel and available in either blue or nickel finish and both were available with either square-butt or round-butt grip frames depending on the barrel length and vintage.
Really, the M19 was based upon, and is more similar to, the M14 and M15 than the M10. The true .357 Magnum version of the M10 would be the M13 which differs only in its chambering, cylinder length, and range of barrel lengths and styles offered (the M13 was available, AFAIK, only with a 3" or 4" heavy barrel). Indeed, it takes a very keen eye to tell the difference between a M13 and a heavy-barrel M10 of the same barrel length without looking at the markings.
Also, while newer S&W .38 Specials may or may not be heat treated the same as the .357 Magnums, the older ones most certainly were not. I rather doubt that S&W would confirm that newer guns are heat treated the same as that would almost invite some idiot to attempt to shoot overloaded ammunition and blow up a gun. While I know you have no interest in doing so, it is worth mentioning for the benefit of whoever else may read this thread that under no circumstances should one attempt to rechamber a M10 to .357 Magnum or load .38 Special ammunition beyond SAAMI .38 Special +P limits for use in a M10.
Finally, as to the M19 or other .357 K-Frame (Models 13, 65, and 66) digesting a steady diet of Magnum ammo, it depends what sort of Magnums you're shooting. As has been mentioned, the problem area of K-Frame magnums is the forcing cone. In order to clear the yoke, the forcing cone of a K-Frame with a one-piece barrel has to have a flat spot ground onto its outside at the six o'clock position making it quite thin in that area (this is a trait unique among S&W frame sizes). This isn't really a problem with .38 Special ammo or .357 Magnum ammo with bullets over 140gr (158gr is the standard weight for .357 Magnum and pretty much all that was available in the 1950's when the original Combat Magnum was introduced).
The problems came in the 1970's and 1980's when lighter, faster 110-125gr bullets became popular in .357 Magnum. These loadings, if used in large quantities, are known to erode and eventually crack the forcing cones of K-Frames in the thin six o'clock position. The reason for this is fairly complicated but, in a nutshell, the combination of lighter, shorter bullets and larger amounts of slow-burning powder means that the bullet leaves the case when a smaller proportion of the powder has been burned and that it it does not completely "seal" the chamber throat. This means that extremely hot gas and burning powder can flow around the bullet and into the forcing cone thus causing accelerated wear. A better, more detailed description and explanation can be found in the following article.
http://www.gunblast.com/Butch_MagnumLoads.htm
The primary difference between the K-Frame and the L-Frame is that the L-Frame is slightly taller and thus has a taller frame window. This allowed the use not only of a thicker forcing cone, but also one which didn't need a flat spot at the six o'clock position. It is also worth mentioning that J-Frames don't have the same forcing cone problems as K-Frames even though they're smaller and have even thinner forcing cones. This is because J-Frame forcing cones don't have a flat spot and because they don't protrude as far into the frame window and are thus better supported by the frame itself.
So basically, K-Frame Magnums can be quite durable if shot with .357 Magnum ammo using heavier bullets at more moderate velocities. If, however, one insists on shooting light bullets at high velocity, then a K-Frame is more likely to have issues than other S&W .357 Magnums.
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