I prefer the Mossberg for reasons that have been posted, ergos, the tang safety being one of 'em. I'm just guessing you're a righty, so that's not so much a big deal.
I hunted with my 500 this morning. That thing is smooth and sweet after 20 years mostly shooting ducks and geese. It shoots where I look. I had to shim the stock to get it to fit, but that's probably something I do with an 870. I'd buy another Wingmaster at a gun show or something, but honestly, I like the Mossberg better. That's why I've switched. BUT, it's a personal preference.
One thing, the Express models aren't the guns out of the box that a 500 is, IMHO. Their finish is what I most dislike, friggin' rust magnets. Net wisdom (an oxymoron?) says they have had some QC problems. Take that for what it's worth, about nothing. LOL If I got another 870, it'd be an older Wingmaster, not an Express. If I get another used pump, though, first up would be a Browning BPS and perhaps second, I'd like to find an AFFORDABLE Ithaca 37 featherweight...another oxymoron.
My buddy's got a favorite shotgun I bet you'd like. He uses it for everything from doves to geese, well, except that he got a new BPS 10 gauge this year. It's a 24" barrel 835 ultimag. The shorter barrel on the ultimag sort of offsets the added weight of the gun due to the 3.5" 12 gauge capability. It's a nice gun that can do ANY sort of wing shooting. You won't see one on a trap and skeet range, but it's an awesome hunting tool.