Just going off of the original post, buying a reliable, affordable shotgun for a friend, not knowing if shooting for fun or defense or both, and, after shooting both, I would have to say the Mossberg 500.
The 870 is just as reliable, and some say built better, but your going to pay 50-100 extra bucks just for the name Remington to be stamped on it.
I carried a M590A1 (basically US Army issued Mossberg 500) and when I got out the first firearm I bought was a Mossberg 500 Special Purpose, which is a fine choice if you are a fan of synthetic furniture on firearms. I have fired over 1000 rounds through my 500. Everything from high-brass buck, slugs, field loads, flares, dragons breath, flechette, bean bag rounds, bird bombs, you name it, and have never, NEVER had a mis-feed.
Also, in my opinion, the Mossberg 500 is easier to reassemble after cleaning.