HSMITH has got it right on the nose.
Usually if you are using steel shot you want to stay away from full choke, as mod and I/C will deliver better patterns on most guns. I have very little experience with Hevi-shot, but from what is known it is by far the best choice out there for waterfowl if you can get over the sticker shock.
Bismuth is another choice that is better than steel, but not as good as Hevi-shot. It will generaly pattern out of a full without too many scattered flyers if the shot is small, 5's and 6's for ducks. 6's and 7.5's for Hevi-shot, and 2's-5's for steel.
If you can figure out what your maximum shot will be, set up a pattern board at that distance and try each choke for best performance. Don't try to reach out too far with steel, stay inside 40 yards, closer if possible.
