I used to run 400 rounds through a Mossberg 500 almost every weekend. This was back when WD 40 was considered the best gun lube. I would just field strip the gun, spray it with WD40, wipe it down and reassemble it. I shot a lot of green dot powder at the time. Green dot leaves a bunch of a cork like substance inside your gun. WD40 cut right through the cork and dirt with no problems. Today CLP does a much better job of busting through the crud.
The only difference in reassembling a 500 and a 870 is you can stack the parts through the bottom of the receiver on the 870. The mossberg 500 you have to slide the parts through the front of the receiver. I don't see either gun as difficult to maintain.
One thing bugs me really bad about the 870 express models is the rib seldom lines up with the receiver. That would drive me nuts. I am not saying that mossberg is perfect but it's not difficult to find a 500 that the rib lines up with the receiver.
I have a 9 shot maverick 88 that people would consider as a tactical shotgun. I bought it for deer drives. I have a 1600 lumen light mounted on the Maverick. That gun has been through hell and it still looks pretty good. I highly recommend the Maverick 88 with an extended magazine. You can find them new from $230 to $260.
People say a 28" barrel is hard to handle in tight spaces. I seen lots of hunters navigate brush that constantly ripped at your clothes and get off some very quick shots with a 28" barrel. Walking beagles will not push a rabbit out of heavy cover. Hunters get right in brush so thick the dogs ears need attention after the hunt. They have no problem getting off a very quick shot with a 28" barrel. The only place a 18.5 barrel has an advantage is inside a vehicle.