Timely post.
I just scored a 645 in really good shape. I was actually looking for a 4516-2... or maybe a 4506-1... but the 645 came up on Armslist for a good price, so I said what the hell. The gun is just... so freakin cool. And so unquestionably reliable. I didn't shoot it great the first time out. The trigger seems to break really far back, and the grips are a little narrow... I might try some aftermarket options (Hogue wood grips came highly recommended).
No experience with the 4506. But... from what I've read... the 4506 is basically an updated 645. Whereas the 4506-1 is basically a 1006 chambered in .45. They eliminated the lightening cuts on the frame, and beefed up the slide (I think) to handle the 10mm... and just decided to use that beefed-up version for their full-sized .45. Also, the trigger guard is rounded.
I love the DA/SA Smiths. I've always thought they were underrated guns. I wish Smith would resurrect them in some form; they currently don't have a DA/SA offering.