If you're thinking of shooting 3-gun - and the regs allows - I'd grab a semi. But, if we're sticking with the 870 vs. 500 discussion, I'll say this: You can't go wrong with either. Pretty well every point above can be considered valid.
The Mossy: Has better layout in regards to release and safety (as long as you don't run a stock with a pistol grip). The dual extractors are great, but no necessary. And yes -- they are pretty well bomb-proof, especially if you grab one with the heavy barrel (590a1). They are heavy, and seem to profile a little larger than the 870. Despite having an aluminium receiver, I believe that it's a little bit larger. People usually praise the 500 series because there is no tilting ramp in the loading port; it's not a deal breaker, it's just different. Keep in mind that that are going to be JUST a few less accessories and variety.
The Remington: Is one slick piece of machinery. Though the safety and release aren't as well laid out, they can get pretty darn quick. It's true, the release on the 870 can be a little sharp...but that can be smoothed out with time, or elbow grease. The gun balances well, and they typically won't die. I know people that have 20 years on their 870s, with wet-season hunting, and more rounds than I can count. The complaint I hear is 'The new Express guns aren't smooth at all'. Truth, but neither was that Wingmaster 20 years ago. I purchased another 870 a couple of months ago, and it already runs about as smooth as my old Wingmaster. It all takes love. But I went off on a tangent there. What I was trying to say is that I hear junk about MIM parts, and the fit and finish...never about them quitting, or an extractor actually dying. That tilting ramp I was talking about - that some people hate - is there partially to keep all of the junk out of the gun.
At the end of the day, they FEEL different. That is the most important part. I would get yourself down to a shop, and go hands-on. You will know which shoe fits. I have both 870s and 590s, and I will say that I like the feel of my 870s more. That isn't to say that I don't like the 5's.