Despite what modern day experts say, the old 26" Skeet guns handle best, especially in a pump. Learning to shoot Skeet with a pump will hone ones ability to operate the slide to a high degree, to the point where observers can't detect any movement in the forearm hand.
I like the M37 better than the 870, as a gun, however the 870 is more user friendly on the skeet field(as is the M12 or M31).

Ole Cowboy