Here, the Academy has 870 express super magnums for $277.87 and mossberg 835 for $338.86 (in shadow grass). I think you have a little room to negotiate prices down some. And the 870 listed here will shoot 3.5" magnums according to the ad.
I have never used a Mossberg, so I can't comment there. Remington has made the 870 for about 50 years, without much change. There's a reason for that. The 870 is about the most durable gun I can think of. It is next to impossible to make one not function properly.
Here is the link if you want to see the ads:
http://www.academy.com
The guns are listed in the weekly ad a couple of pages deep.