I have both a M870 Remington and a M500 Mossberg. I also don't see much difference in operation and could live with either one. I also prefer the
tang safety of the Mossberg as well.

However, I feel the Remington M870 is a better long-term gun as the
all steel contruction seems to more durable. I feel the durabilty is of
particular importance if the gun is misused and/or neglected in a SHTF
situation.

I realize the M500 have some military contracts but if you have something
that you have to use with in an adverse situation I think the M870 would be the better of the two. For most normal shooters uses, however, I would imagine either gun would serve you well.