I'm going to have to disagree w/ you here, sb. Choke diameter is very important, but there are so many other factors involved that just placing a tighter choke in a barrel does not guarantee better patterns.
Some of those factors-besides constriction- include: shot composition, size, barrel length/velocity, forcing cone taper, wad design and backboring. Not to mention temperature and altitude.

As to all modern shotguns being able to use steel, keep in mind I stated alternate shot, which also includes hard compounds like tungsten.(Between Rem. and Mossberg) Remington states on their site that Rem chokes should not shoot steel shot larger than #2 through full chokes and that goes for large Hevi shot, as well. Mossberg Accu chokes don't have this restriction. Advantage: Mossberg.
I like the 870. It's perhaps, overall, better than the 500 but either will give their owner long usage and great performance.

Anyone with interest in this can go to Remington's site and type into the search engine: Can I use my current Remington shotgun/choke with Hevi-Shot or Wingmaster HD loads.