I have several of both. They are both good, but for me, the Mossberg is a better gun. It is not as traditional as the 870 in that there are stampings and aluminum and plastic parts, and I think some people are put off by that, however functionally I think the 500 is better.
Think of it as comparing a 1911 to a Glock. They are both good, traditionalists prefer the 1911, but out of the box, the Glock is probably more reliable. With tuning they are probably equal, but the Glock (and model 500) gets there cheaper.