Either a Remington or Mossberg would be fine. Both are very good shotguns. You are unlikely to wear either out.
Handle them both and see which one fits you better. Also try out the controls and see if you like one better.
You can't really put smaller gauges into a 12 gauge. They are too small to function. There is such a thing as a subgauge insert, but they are expensive and really only an option for a single shot or double barreled shotgun.
It is possible to fire a shorter shotshell of the same gauge, however.
Make sure you see what other shotguns the store has. If you like one in your price range, feel free to ask us about it. It might also be worth it to check pawn shops.