Can't go wrong with the 870. Remington makes a bazillion of 'em, & they are very versatile, and you can easily customize it as there are alot of parts easily available. I think you can still pick up a decent one for around $300. I'd advise the 12 gauge. Rounds are cheaper & more readily available, & you can get lighter loads for the smaller birds.

I personally use a Benelli SuperNova 12 ga. pump. Set me back $400, & I can put 150 rounds through it @ the sporting clays field & barely feel it. Unfortunately, prices have come up since then...