I know this thread is old but...
I would say turkeys because ducks are mean and messy. I was going to get some ducks a year ago but one of the workers at tsc said they're really mean and can attack chickens and even people. I heard about one seriously hurting a young rooster.
Ducks fly south for winter so you wouldn't get as much time with them as turkeys and ducks can bring back diseases from their trip down south and give it to your chickens when they come back up. Ducks also poop everywhere.
Turkeys do eat slot but there much friendlier and will act as better pets and may become friends with your chickens.