The Mossberg 500 would be your best bet. Like said you can get them in combo packages for under 300 bucks sometimes. Comes with a bird barrel and 3 chokes and a fully rifled slug barrel. If you plan on using a scope get a cantilever barrel. That way you can mount the scope right to the barrel, not the receiver. It will most likely be more accurate that way, and when you want to swap barrels the scope comes off with the slug barrel.

You can get a Remington 870 the same way, they cost a tad more though and you only get one choke with the bird barrel.

Paul