Eldeguello has it right. Use your 870 SM for ducks, and for upland game get yourself a used but good side-by-side20ga. I have an AYA (Spanish brand) that I love hunting with. But there are many other brands that are excellent such as Parker, Fox and others. Lots of good quality examples can be found gathering dust in gun shops because they went out of style.
Or, find an over/under you like and use that. Browning Citori's are great guns, although a little pricey when new.
If you shoot your 870 really well, and you want to stay with a pump, get a Wingmaster 20ga, or an Ithaca model 37 20ga. Both good guns too.

Best of luck, and have fun shopping around!