I have a Mossberg Ulti-Mag. I have had it for about 20 years and I have no complaints. I was originally leaning towards the 870 but back when I was shopping the 870 wasn't available chambered for the 3 1/2" magnum shells.
Of course, in the ensuring 20 years I have never once cycled a 3 1/2" magnum through my shotgun, although I have fired thousands and thousands of regular shells. Maybe I just lucked out, but my Mossberg has given me no trouble whatsoever and is a great gun to shoot.