MetzerFarms.com does a lovely comparison of ducks.
Also consider what you find good in a pet. Do you want to cuddle - because then, you may only want one. Do you like to watch them swim? Do you want to watch them run through your lawn eating bugs? Do you like to clean a duck toilet - because that is what a small pool becomes quickly.
Mine are bug-eating lawn ornaments. The Pekin drakes love to hang with people, but I never really handled any of my birds. The flying Mallards are pretty, and love to dive underwater and swim. They are just learning to fly.
I just ordered 8 Indian Runners because they are supposed to run fast through the gardens without tramping plants. (My fat Pekins stomp everything down and want to stay inside the fenced yard and gardens. My chickens and Mallards go out in the open yard every day.)
I like the pretty colors of ducks. I believe duckweed and other plants can make their plummage more beautiful.