I'm aware that the thread is about secondary weapons, not method of carry for your secondary weapon, so I suppose I'll preface the following remarks by saying that I vote in favor of a 642. It's relatively light, though with enough weight on it so that 38+p doesn't smart that much.
In keeping with the fact that this thread is in the semiauto forum and not the revolver, then let me mention that there are the Glock 30s and the Kimber ultra carries. Still small and relatively light, with the added benefit of carrying more and of a larger caliber.
Regarding carry, I agree with Fed168; I keep my 642 in a pocket holster and it's a remarkably fast and unexpected draw if you happen already to have your hand in your pocket. If you're being mugged it's an easy thing to say that you're just reaching for your wallet (though there is the frequent argument that you should give up your wallet before you pull your weapon). You do, however,mention "duty", so I assume that concealed carry isn't your primary concern. There are fewer excuses to kneel down and draw from an ankle holster, though I remember reading about a police officer who was surprised, disarmed and was about to be shot when he asked, as his final wish, to pray. Laughingly, the BGs let him, whereupon he knelt, got his j-frame from his ankle holster and shot them both.