The answer is which ever one fits you best and which ever one you like best.
I've owned a shotgun or two and can tell you a few things. If you want an 870 Express, go for it. Pay no attention to those who tell you the Wingmaster is a better gun, their the same gun. One is made to hunt with, one is made to show off. The old wingmasters are not the same as the new ones, back in the day the old Wingmasters were the cheapy gun equal to the Express. And back in the day they didn't cost $800 even though guys who own the old guns like to compare them to the new ones.
I've owned several Express models in 3 inch and 3 1/2 inch. First, you don't need a 3 1/2 to shoot a duck. I own a few decoys but rarely use them. I'm too much of a simplton to get that elaborate and just too lazy. I like to pass shoot as the birds fly over and that means longer shots. Your shell choice and shot size are far more important then how long your shell is. I shot many, many birds before I owned a 3 inch gun and they all fell down the same just like they did with 3 or 3 1/2 inch shells. Now I like to shoot 3 inch only because it's easier to find BB shot in that length, my prefered size, though 2 3/4's can be had.
As for the gun, my first choice is the 870. I've never really had any serious issues with any except one didn't like Federal Black Cloud shells. It ate everything else fine. I like them because they fit me, but you may be different. The only other modern pump I've owned was a Winchester but it never really did anything for me. My best advice to you is to buy a synthetic stock as you will beat it up. I've dragged my guns through mud and cat tails, I've made them swim, sit in the rain and snow and ride on my dashboard. A synthetic will handle the abuse better then wood. As for the 887, I've never shot one but I like the concept and if the gun is good it would be the perfect waterfowl gun.