Info I've had on Super Mag glitches has been second hand, but here goes...
Since the Super was not purpose built, the mod of the ejection/loading port left a long enough channel that some torque and binding happens on SOME Supers.The Supers have that extension behind the bolt,IIRC.
This can lead to or be found with feed probs. A smith can rectify this, or possibly just more breakin.
I can't help you on picking one of those shotguns. But, if I had more than a few 870s, I'd be tempted to try something different. Then after dealing with the something different a while, I'd probably trade it off for an 870.