It really depends on how much you want to spend. But I definitely wouldn't get a red dot type. IMO the dot would cover too much area at your 200 yard max range. Bushness makes a decent scope but in today's market there are better. A nice Nikon Pro-staff is a good choice. But even better would be a Leupold Ultimate Slam scope if you can find one.

Why do you like the 870 over the Mossberg?

Nothing wrong with a Mossberg 500, they are very will proven over the years.....Today's Remington firearms are junk, older ones are great !!

what gauge are you getting?

If it's a 12ga stick with 2-3/4" sabot slugs
If it's a 20ga stick with 3" sabot slugs

You will need to try different brands to see that that barrel likes, but a good starting point would be Remington's Accutips, and the new Remington/Barnes sabots

Scope wise, no need to buy a "SHOTGUN ONLY" scope, purchase a rifle scope 3-9x40....I'm a Leupold fan and have them on almost all my slug guns and rifles....If on a budget a good scope is the Bushnell Trophy, it's clear, and can't go wrong for the price

The few Mossbergs I have handled were "sloppy". The parts seems loose and ill-fitting. Guns were also heavy.

Remington 870 are the tried and true pump shotgun. Very, very dependable.

get a Savage 220!

For a long shot over 100 yds, I would definitely get a scope. I have used shotguns with sabots before and I never shot one i thought I could shoot accurately at 200 yds. Luckily in my state you can use a muzzle loader or shotgun so I use the muzzle loader now. Much more accurate especially at distance and I only need one shot anyway.

What did you choose?