I've been using 870s for over 30 years and never had a problem with them so I would get the 870 Express. Never owned a Mossberg except for a 144LS rifle.
I don't understand this talk about 870 Express guns rusting. Mine was bought in the late eighties and has been used a lot in the field yet over the years I have only found a few small specks of rust here and there. No more or less than you would find on any hunting rifle or shotgun regardless of the type of finnish. Maybe there is a problem with the newer made guns but then again I never left a gun outside for 3 days either.
