I will concede that the 870 is a fine pump shotgun, but it just doesn't have the "cool" factor of a Model 12. The Model 12 was probably the most precisely machined from the best materials, closely inspected, and in some cases, hand fitted pump gun ever made. They dominated the trap and skeet scene for years. I guess there are still a bunch of old geezers like me who are still pissed off at Winchester for exterminating a lot of fine guns in that horrible year, 1964.
Part of the appeal of the Model 12 is that it represents an era of mass production in this country that won't (can't) be repeated. Everything on the gun is machined steel, including the lifter. No stampings anywhere including as far as I know the action bar. When I took my 16 ga. )made in 1915) apart for a cleaning after I got it, I found the lock as solid and nicely finished and put together as all but the highest end double guns. How they made a gun that nice requiring that much machining, even given a lower wage structure, is still baffling.
The 870 represents beginning of another era in manufacturing developed in response to the exigencies brought on by WWII, when the huge scale of production of arms, vehicles, etc. necessitated for one thing the perfection of stamping processes of high quality components. It also benefited from the collective knowledge of half of century of experience with pump guns--one reason for the two action rods.
Shotgun handling dynamics are critical to shooting flying and are individual and subjective. Some prefer the 12, some the 870. I've shot 870s, in 12 and 20, a fair amount. Neither sings to me like my 16 ga. Model 12. I really think they hit the sweet spot with the 16, especially the older ones with the corn cob fore end. I've got a pretty nice english form SxS with excellent balance and handling qualities, and the old M12 handles just as nicely. This is a gun used for upland hunting--handling qualities, balance, would be different on a target or duck gun.
The trend for the last 20 years in the upland community has been toward lighter more dynamically handling guns, and the sub gage model 12s and 37s are very popular now.
So to some degree the 870 vs. 12 debate is a matter of nostalgia and aesthetics, to some degree a matter of where you approach the issue as far as intended use of the gun.