I'll throw in with the 539 vote... I stumbled onto a NIB 559 last year, I like it a helluva a lot better than the 459 I had.
The 59's had some problem with frames cracking near the slide stop lever hole to the slide rail... the 2nd gen addressed that.
Also, the steel frame does add weight and helps to buffer the 9mm recoil.
As other have said... the steel frame versions of the 2nd gen pistols are found in much less numbers.