Not sure if it's worth anything to you but I've flip flopping between an 870 and a 500, back and forth, forth and back. After watching a pretty thorough review of both on youtube by someone who actually takes the guns out and uses them I ended up leaning a bit more toward a 590a1. the reviewer was very pro 870, actually 1300 but that's another story but his bullet points, to make a pun, on the two inched me toward the mossberg. I simply liked the ergonomics of the controls better, the thumb safety, the pump release and the raised carrier when the gun is bolt forward meaning you don't have to fight against the carrier to reload.
it was finally settled yesterday when I stopped at a gun store for something else entirely and to see if they had any 870 tacticals in stock, no one ever does, but lo and behold they had the 590a1 and in my desired configuration (8+1, ghost rings, standard stock and corn cob pump). After shouldering it and some sight drills, it left with me.
I'll be at the range tomorrow but today cycling dummys through it and feeding them into it from speed clips, I have to say I'm happy at the moment with my choice.
The 870 is a very very solid choice and all I can really say is shoulder them both in the configurations you like, if you can find an 870 that is, literally there's nothing around herein an 870 but turkey guns but you can fondle a metric asston of 500's (not 590s).
"I am Man, the pointy tip of the food chain! Gaze upon my opposable thumbs and tremble!"