I used a 590 M last year at the local 2-3 Gun events.
I now have a 870 DM and like it quite a bit more.
While I prefer the standard Mossberg controls ( Safety and Action Release ) I found the Magazine release button behind the magwell kind of awkward.
Also the front of the magazine has to index at the front of the magwell and then you rock it rearward to lock it in place. I found that to be sometimes awkward as well when reloading on the move.
The magazines are very solid and well made but ridiculously expensive (and way too fat to store in most pouches and quite fat in the hand. I used 10 rounders and they are quite heavy and the balance of the Shotgun feels rather odd with 10 rounds hanging off of it. Cant imagine what a 20 rounder would feel like!
But the thing I really dislike the most about the Mossberg magazine is the inability to accept any 2-3/4 shell. It's fine that it doesn't accept 3" shells but 2-3/4 shell vary in actual length and not being able to grab any box of 2-3/4 shells and run them is a hassle.
The 870DM magazines will accept up to 3" shells so I have never had an issue feeding any 2-3/4" shell, even the ones that measure closer to 2-7/8"
The magazines are so much easier to store and handle while on the move, (which is good cause obviously you will need to carry more of them with only 6 round capacity)
I much prefer the DM magazine release lever also. It is in the front of the magwell, so just get a good grip on the mag and lever and yank it out. Then jam another one in in one straight motion, no indexing and tilting required. The 870DM with a full magazine feels much more natural also.
And lastly the price of 870DM magazines are much more affordable @ $45 each. I paid $169 for a 10 round 590M magazine. So you could purchase 3 DM magazines holding 18 rounds total for the price of one 10 round 590M magazine. If one gets damaged from dropping it or something it wont cost as much!
Reliable has 870DM magazines in stock!
https://www.reliablegun.com/en/remington-shotgun-parts-remington-870-dm-extra-magazine-6rds