You'll find more options, and better prices with an 870. The Mossberg 590 is a well made gun that was specifically designed to meet military specs and addresses the shortcomings of the 500. But they are pricey.
The 870 met military specs, but they didn't even enter the competition for the military because they knew Mossberg would underbid them. The 590 was the only entry to the competition a few years ago. They met the requirements and got the contract.
There are other good options such as the Browning BPS and Benelli Nova. But availability and price still go to the 870. I know you don't want to spend a lot of money, but I can only go so low on price and recommend something, and I still keep coming back to the 870.
IME the "issues" with the 870 are overstated. Virtually all of them are related to cosmetics and aesthetics. They made some that were not well finished, and some that needed minor polishing or extended break in. But the design is solid. None of the complaints about them related to reliability.
And the new RemArms guns have impressed me enough to buy one. It was $500 well spent and that is the least I'd spend on a new gun if it absolutely positively had to work, no excuses. I haven't priced used 870's in a while, but the days of a $200 used 870 Express are long gone I'm afraid.
That might be an option as long as you understand what you're gaining and losing. The main advantage of a 20 is a 1 lb lighter gun that is easier to carry around all day. And there is some overlap in the amount of shot in some 20 and 12ga shells.
But you can always put more, and bigger pellets in the air with a 12. That's everything from birdshot to buckshot. In equal weight guns a 20 will recoil less. But in comparable guns a 20 is about 1 lb lighter. So those reduced 20 ga loads end up recoiling pretty much the same as 12 ga loads.
And 12 ga ammo is a lot easier to find and often at better prices. Especially when talking about slugs and buckshot loads.
