Here's a combo that, for around $400 new, will do all you want:
http://www.remington.com/products/firearms/shotguns/model_870/model_870_express_combos_specs.asp
28" barrel for duck hunting; 20" barrel with rifle sights for slugs and buckshot.
Or get the version with a rifled barrel, and buy a Mossberg 18" barrel made for the 870 ($95 from Cabela's) and you have an even better combo, though it will cost more.
Or one of these, plus an extra 18" barrel for even less:
http://www.mossberg.com/pcatalog/model500_1.htm
I love my 870, and a lot of others love them, too. But the Mossberg is also a good choice.
Shoulder them. See which fits you. Work the slide and see which feels best to you.
My take:
870 has a steel receiver and a very smooth action if oiled a little. It's easy to clean, if you feel the need to clean it much.
500 has a prettier finish and a nicer rib, and is more rust-resistant than an 870 Express.
Safety position is personal preference. I like the 870 safety, which is selected easily with the trigger finger without moving the hand, but the Mossberg safety is better for ambidextrous or leftie use, and for those used to double guns.