I, too, am a Rem 870 devotee but to compare it with a Win Model 12 is like apples to oranges insofar as handling is concerned.
In 69-70 I had a job pulling/setting skeet trap at Roberts' Shooting Park (on Roberts' Dairy Farm: we used to have to pick up good used clays from the alfalfa field in front of the traps after the manure spreader came through earlier that day) in Elkhorn NE for $1.50/hr (most fun job I ever had; it is now the site of a Community College thanks to the extreme expansion of Omaha).
A co-worker there had both a Model 12 20 gauge short barrel with a Cutts Comp (man, was that ever LOUD!) and a Model 42 .410, and I shot skeet with both of them many times before I got my 870 12 gauge skeet gun. The 12 and the 42 just feel differently in the hands compared to the 870, but I very much liked both Winchesters. I just didn't have the money for a Model 12.
I have never owned a Model 12, but my Dad had one dating from ~1932 that my Grandfather won in a punch-card game in Detroit back then. It was a 16 gauge 28" full choke, field grade with the corncob fore end, and had seen plenty of use judging by the worn bluing on the gun. Dad was a Herter's catalog fanatic in the 60's-70's and ordered a Belgian Blue kit (hot water bath) from them and also ordered a blond birds-eye maple buttstock and fore end from either Bishop or Reinhart Fajen. One long summer afternoon he and I reblued the gun over a double-burner Coleman campstove in 1970, after he had previously polished all of the metal. He had already sanded down/finished the wood. He was proud as punch with the finished product.
I came home on leave (USAF) in 1972 just before opening day of pheasant season, and my Dad offered the Model 12 16 for me to use, so I got a resident hunting license that afternoon and off we went the next morning to a 2,000 acre milo/corn farm in David City NE. The longest shot I ever connected with was using that Model 12 16 gauge using his favorite Remington high brass Express #7-1/2 loads. It was paced off at 60+ yards on a rooster (classic skeet station #4 low house bird, only nearly 3 times the distance). I outshot my Dad, 2 birds to none, but he was using his newest favorite Ruger Red Label 12 gauge, but he never could shoot it well. He was getting older and I think he liked carrying it in the field better than the longer guns.
To finish, although I like the 870 better, there is nothing at all wrong with the Model 12. As Virginian has stated, the Model 12's demise came about because it was no longer economical to produce.
Here's my 16ga upgraded 26" vr mod.
https://thefiringline.com/forums/attachment.php?attachmentid=104635&d=1489282719
That, sir, is a closet queen and should remain so forever.
I don't care if it is not a factory original: if I owned it I would only show it to select friends, and then, maybe... Beautiful!!
I have enjoyed every post on this thread, probably because all of us here that remember shooting/owning a Model 12/42 are pretty much long in the tooth like me.
Thanks to all for a great conversation!
Jim