For more decoying ducks like Puddle Ducks (Mallards, Wigeon, Pintail, Teal, Gadwall, Wood Duck) where you are taking closer, more passive shooting and the birds are trying to sit in the decoys I use a Modified choke, but when I am hunting Divers on the Columbia River, where they are flying by fast the majority of the time and are more pass shots a little farther away I use a Improved Modified inbetween a Mod and Full. I think it gets my pattern a little tighter a little farther away. but it is also a confidence thing on what you feel more comfortable with and what you shoot better with