I didn't vote, because there wasn't a selection of new or used.
I own 2 S&W revolvers; a 6" 686-2 that I bought new & a 4" 57-1 that I bought from a shop that had it on consignment a few years ago. Additionally, last year I bought a new M637 for my daughter & a new 3" M60 for my wife.
In Rugers, I have a SS Super Blackhawk, a 2¼" SP101, & an old SuperSix - all bought used.
In contemplating all of these guns, I would cast my vote with those that prefer a 686
if it is a pre-lock. I really don't like those locks & stalled for weeks trying to decide whether or not to buy my wife & daughter guns that had them. In those cases, the only alternative was Taurus. While I don't have a problem with Taurus, the tales of Customer Service problems & my dealer's input ended up tipping the scales to S&W when it came to buying guns for family. (Had the gun(s) been for me, it
may have been different). The trigger on my 686 is like butter, & that is just from firing. It is accurate too. I did have to send it in for warranty work as the cylinder developed too much end shake, but it was fixed & returned without issue. (At the same time I also had the insert in the front sight replaced & remember a $25 charge for that. I didn't mind paying, but thought they could have picked that up for me...
)
I haven't handled a GP100 but if I were buying new & it fit my hand & application, I'd probably go with the Ruger. As I'm sure you are aware, Ruger generally builds revolvers than are more "chunky" than S&W & appear to be stronger. Although that's another reason my wife & daughter now have S&W revolvers (they are slimmer & weigh less than the SP101 & therefore are easier for them to handle), I think the Rugers probably are stronger. In the examples I own, the factory trigger pull on the Rugers is not as smooth or light as on the S&Ws. I also own a 5" MKII semi-auto pistol & it had to go back to Ruger; it also was fixed & returned to me without issue.
Have fun shopping.....