Recently I bought each of the shotguns you are asking about during a black Friday sale (590Tactical & 870Tactical). I can offer some experienced impressions I witnessed with each of them sitting side by side. My intention was to see for myself after all these years how they compare and what improvements I might make without spending any of my retirement money other than to purchase the shotguns. Since then I have sold them to folks I know. Here are my observations...
1. Both examples were the tactical models with 18.5" barrels and synthetic stock and forend. Both cycled as they should out of the box.
2. Both had decent fit expected for the tactical models with a few exceptions I will mentioned below.
3. The barrel on the 870 had a nice shiny bore as expected.
4. The barrel on the Mossberg was unfinished inside and had the same rough surface as the outside of the barrel. MB customer service responded & said that was normal! Subsequently I shot a full box of ammo through it to get some heat in the barrel and see if it would be hard to clean the plastic wad material out of it afterwards. Sure nuff! Much harder to clean than a polished bore.
https://www.thehighroad.org/index.p...r-experienced-mossberg-500-590-owners.913372/
5. The Mossberg required greater effort to cycle the bolt than the 870 by several pounds of effort. Most of the difference was in the very first inch of the cycle when the bolt was recocking the hammer.
6. Each of them benefited from a slickening process by polishing some internals with flitz and a polishing bob. The Mossberg benefited the most, and dropped the amount of effort required to cycle it by 4 pounds. It still remains more effort than the 870 before and after slickening. The 870 was measured at 5lbs to cycle. The 590 was almost 10 lbs to get it started.
7. The controls location preferences are up to the user. I did install the metal safety actuator on the MB but really didn't see much difference in the effort to activate the safety.
8. For me the bolt release position on the 870 feels more natural.
9. The 870 is easier to disassemble, but neither should be considered a deal breaker for that reason alone.
10. Some folks prefer how the shell carrier operates on the Mossberg. It is a nice feature but not a deal breaker for my uses. Either works fine.
11. Both have a reputation of reliability, and I saw no reason internally that would doubt the continuance of that same mechanical performance.
12. After a shooting session with both shotguns, MY choice was the 870. The ergonomics fit me and it just feels lighter and more handy to me.
These were my observations on the two shotguns. Shotguns are not new to me. Currently I own 15 of different vintage and models. I have owned several 870's through the years. However, I had never owned the Mossberg! I wanted to see the differences and what I had been missing. Growing up in the 1960's, the Mossberg was considered a cheap shotgun to buy if you couldn't afford a better one. After a few years Mossberg made a few design improvements to the 500 and history has been made since. Choose what you like. Either will serve you well. Try to lay them side by side in a bog box store and see for yourself. Good Luck!