If you can afford it join all 3 (you forgot Waterfowl U.S.A.).
If you can only join 1,you have to decide what you want to support.
If you want to preserve and add to habitat nation wide , thats what D.U. does.
If you want to see more research done that helps improve the waterfowl population, thats what Delta does.
If you want to see projects done in your home area , thats what Waterfowl U.S.A. does.
All are good orgs. and do their "thing" well,you just have to decide which one does what you support the most.