Over on Shotgunworld this in one form or another is about THE most common question asked. The answers are always about the same and the discussion usually devolves into a Ford versus Chevey versus Whatever kind of argument. 870's and Mossberg 500's are 1&2 in popularity and available accessories. The Nova people either love or hate and there's a small cadre of Winchester 1300 fans. All good guns, all roughly the same price. There are also die hard used gun fans that favor field grade model 12's or Ithaca 37's and there's a host of miscellaneous other good deals out there. My personal favorite is a JC Higgins model 20 from about 1955, sold by Sears, made by High Standard, decent wood, smooth as silk and points like a dream - $150.
Point is within the that price range you have many good and pretty much equal options. So take some time, shoulder them, try them out if you can and get the one that fits best. They're all time proven designs so it's hard to go wrong with any classic pump, especially if you stick with the name players (ie avoid some of the really cheap imports which can be very hit or miss). If you go used, you might want a gunsmith or experienced friend to take a look first.
Find the gun that fits and enjoy. Don't get hung up on small differences