Although I don't personally own an 870 SuperMag, I've heard and seen extraction problems with them. My SuperMag is the 11-87 auto, and the only problem I had was a broken extractor that Remington replaced for free.
The next question is why the 3.5" shell. My requirement was using steel shells in waterfowling (geese specifically), so the 3.5" made sense until I started comparing it to the performance of 3" HeviShot shells. Hard to tell the difference, so I could have saved the extra price of the SuperMag chambering and just gone with a regular 3" mag.
The gas gun seems to moderate the felt recoil as compared to the pump, but that is just my personnal preference.