My sample from work of over 150 guns, somewhere in the area of 180 guns (I forget exactly what the inventory numbers were) mostly 500s, some 590s, several 590 14" guns, is that they have proven to be tough, reliable, handle abuse and neglect well, and are easy to detail clean and fix as needed.
Our run big of 500s (bought all on one 125 gun batch purchase) did appear to have magazine springs made from recycled paperclips, and we had to replace them after a few years. I dropped Wolff Gunsprings magazine springs into those guns, and Vang stainless steel followers. Zero issues with feeding after that point.
That Vang follower I can HIGHLY recommend for anyone using a tube fed shotgun.
My experience with older 870s was always positive, however I didn't note them to be better than the Mossbergs. Newer 870s have had QC issues that I have heard of.
The placement of the safety on the Mossbergs is superior IMHO, and one reason why I prefer them.
If you want to add a pistol grip stock then don't pick the Mossberg, that safety no workee when using that type of stock.
My personal 590 is a 20" gun with ghost right sights, it has been very well used in multiple shotgun courses, and has been a loaner to guys at work that needed a gauge to take to a class when the job wouldn't give them a loaner. I've easily got over 10,000 rounds through that gun with no issues outside of out of spec ammo, etc., with only a mag spring and follower being upgraded in that time frame.