Around here, a new plain "Witness" 9MM goes for about $350. The closest currently produced equivelant to the S&W M-19 goes for $150 plus more. So an even trade of a good used Witness for a good used S&W M-19 sounds like a good deal. And between the two, the S&W is probably the more "quality" handgun.
The M-19 .357Mag is basically a slightly beefed up S&W M-10 (or Military & Police) .38Spl that has been in production for over 100 years. Probably the most used Police handgun of the 20th century. In other words, a very refined and proven design. The M-19 came along in the 1960's & 70's in answer to Police requests for the more potent .357Mag round in a realitivly light & compact package.
Some will say the M-19's short coming is that a steady diet of full power .357Mag will cause forcing cone wear, timing problems, etc. However, if the example you have locks up tight, the gap between the cylinder and barrel is about the width of a business card and there is no visable excessive wear - it probably hasn't been shot excessivly. The truth of the matter is though, that most guns do not get shot excessivly (like tens of thousands of rounds). So the supposed issues with the M-19 never had much merit anyway for the majority of users. You will probably find that shooting .38's is going to me the most enjoyable and economical - but you will be able to shoot your heart's desire of .357's without worry. And, if anything does ever need to be fixed - S&W has a good service operation (not necessarily so with EAA). There are also ample gunsmiths and parts around for tuning and customizing if you wish.
Regarding the "user" attributes between the two guns, I think you will come to appreciate and enjoy the flexibility, accuracy and feel of shooting this revolver over the auto.