You are never going to get your eyes pecked at by a duck like you would with a chicken.
Most breeds of duck don't fly, so no wing clipping. Except for bantam breeds of duck and muscovy.
Hardier than chickens.
No rooster crowing at wild hours of the day or night.
No need for a roost, except for muscovy.
But the hens will sometimes quack loudly, only in the daytime except for a rare emergency.
Also ducks are typically not as friendly. There are some breeds that are friendly though.
Their water areas (for drinking and/or bathing) are messier if you aren't careful how you set them up.
More messy poops, but that dissolves quickly with a squirt of the hose.
Not a whole lot of difference otherwise.