Hi Sanna, I have both chickens and ducks in my small garden here, they get along fine together, although there is an issue about cross species mating which should not happen between drakes and hens (it's fine between roosters and ducks, but drakes can seriously injure hens); not a problem if you don't keep any drakes.
For the most part I prefer the ducks in a small setting like ours, as they are not destructive like chickens: they don't scratch and are more interested in bugs and slugs than grass and vegetables--chickens prefer the vegetables. While it's true that ducks can survive without a pond, it's really not fair to deny them something so central to their nature. A small kid's paddling pool is a good substitute. But they are very messy and their water will need to be changed daily. They also seem to lay their eggs anywhere, unlike chickens who will stick to the nest box.
Chickens on the other hand are excellent for tidying up where it's overgrown with weeds, or at the end of the growing season; and will take all your garden trimmings, weeds, and other organic matter and turn it into wonderful compost. I prefer the taste of chicken eggs over duck eggs, but can eat them scrambled together, and duck eggs are very good in baking. Chickens I think make better pets, at least in my experience, as they are more inquisitive and interested in people; ducks are more independent, but both can be very friendly and trained to come when called.