Impressions of any of them?

Pros or Cons to each?

No cons on the 64 & 66. The 617 is a little heavy for a .22, IMO, but it was designed that way for cheap practice for the 686 owner crowd. Should be a damn acurate .22, as well.

I used to have a 66 & like many others, regret letting it go.  

The only con to the 64 is that you can get the 65 and have the 357 option. If all you're shooting is 38, though, thats irrelevant. You'd have a tough time wearing out any of the K-frames with 38 or 38+p loads---but a CONSTANT diet of heavy magnum loads will take a toll on the K's (but this is typically over-exaggerated), and therefore some people shy away from the much easier carrying k-frames for the heavier L-frames, which were better designed for CONSTANT magnum use. IMO, few people ever run enough mag loads thru anything to make much differance.
I have 4 mod. 66's right now, and think its one of the best revolvers, if not the best, ever built.
I recently sold off my rare 3 inch barrel version, because some guy was willing to pay any price,  but i'm sorry I did now.
Had a 617, 6" once--to much gun (weight) for a 22, unless you only shoot off the bench