My first S&W pistol was a nickel 459, I sold it after getting a case of Glockitis and kicked myself until I recently replaced it with the carbon steel frame version, the 559, that I happened onto.
The second gen's internals aren't all that far removed from the 3rd pistols, there is a weight difference and additional refinements, but at the same time many of the parts haven't changed substantially since the 1st gen M59.
The 3rd gen eliminated some of the 2nd gen grip frame by incorporating it into the lightweight Xenoloy grip, which dropped both weight and girth, but the rest of the frame is basically the same.
The slides are more alike than different , the 3rd gen has a different front sight and the barrel bushing is pressed into the slide.
In addition to my 559, I also have 3rd gen pistols and I don't think you can go wrong with a 2nd gen pistol.
Conrad - S&WCA #1830