Taking my beater 870 20 gauge duck hunting this year with a buddy.  River bottom ducks.  Looking at Winchester DryLok 3" #3's, 1 oz.  I plan on ordering a Carlson's extended choke that they recommended.

Thoughts?  Never been duck hunting before, but I'm familiar with the area and there are lots of birds.  Hard to get to unless you walk.

Yeah, I know the season is a ways off, but it will take me at least this long to figure out the bag limits, seasons, etc.  

I typically shoot kent 90% of the time.  Rio has actually patterned very well for me too.  I shoot a kicks in all of my hunting guns typically.  My patterns were always a little funky with winchester (Last time i cut a few open they had a few out of round pellets/abnormalities)

Federal #4 black cloud, close range if you can find it works great on wood ducks and teal.
Winchester #4 drylock don`t pattern worth a damn, open a shell up and let the pellets roll across a flat surface.
They are as out of round as Hevi Shot, not a good thing with steel.
Hevi Steel #1 worked very well for me on early season goose, within 40 yards.

best advice?  Pattern your gun.
Don't get caught up too much in the "premium is automatically better."  Sure Hevi-X and bismuth have better ballistics, but if they don't pattern, you're just punching expensive holes in the sky.
If you do want to shoot the premium loads, you might have to try a few different chokes to get the patterns you want at the ranges you think you'll be shooting.
I've shot hundreds of ducks and geese with a 20 gauge 870 wingmaster and patternmaster midrange choke.  Don't focus on ammo brand, just find a good patterning load of 4's or 3's for ducks, and 2's or 1's for geese and buy a case or two.
My goal for next fall is to find a winchester SX3 in 20 gauge or benelli 20 gauge and shoot more geese with the 20.  My daughter will be hunting next fall and she'll be shooting a youth winchester pump 20 gauge, so just taking the same ammo for both guns will be nice.

A couple years ago I made a frame to staple wide sheets of paper they sell at paint supply stores, a 4x4' sheets worth.  Quite a difference between ammo brands for sure.

Definitely pattern your gun!  Having said that, I've had very good luck with Hevi-Metal 2s, 3s, and 4s, combined with Patternmaster chokes.  20ga ducks in timber I would try a short-range/decoy choke and 3" #3s.

interesting note about 20s.

many manufacturers have side noted or mentioned that 3in 20 ga ammunition is extremely hard to consistently pattern. some double barreled makers just flat gave up and went back to producing 2 3/4 inch chambers. for some reason 3 inch steel 20's are the bad dreams of ammunition engineers.

you might have to try 3-4 chokes and 6-7 loads before you find one you are comfortable with.

when my son was young we put briley's on his 870 youth. I ended up with winchester supreme loads.

Boss shotshells.

Copper plated bismuth.  #5s.