Hi!
What duck breed would you like, exactly...?
Some duck breeds are less messy than others... but with ALL the ducks, you will have to change the water they drink AT LEAST two times a day, because they tend to dirty (all) the water.
(Ducks need to dip the head in the water to clean their nostrils!)
I have Indian Runners...
They are good to have : they walk funny, don't fly (I just had a drake blown away by the wind ONCE, because my department is ridiculously windy), don't eat a lot (no more than my chicken, I believe?), and are not so messy compared with some other ducks... and they lay A LOT of eggs!! (My duck laid eggs almost everyday until their molt!)
(My runners don't even eat a lot of grass - they make NO damage : they prefer to eat slugs and worms!!)
Eggs from ducks are richer in flavor, and taste better - allegedly... but actually, some people don't like these eggs, so... you will have to taste them personally in order to decide if you like them or not.
And to warn you : you really should cook the eggs from ducks before consumption, because they are renowned being particurlaly susceptible to Salmonella infections.
(Make bread or cakes!)
Actually, ducks have some advantages to chickens : if you allow them to swim regurlaly in a pool or in a pond, they won't have lice. Nice, right...?
Also, they are just so beautifuuuuuul.... Their bill are cute, their feet are adorable...! (Look at them SWIM, and you want them into your garden!)
And the white duck are soooooo WHITE (clean!) you can easily spot them at night. That is very good for those who return from work when it is dark, and who can only lock the birds up at night! (No, really...!!)
Another advantage : ducks are fairly more resistant to diseases than chickens.
I never had a sick duck. In fact, I never even had a duck with (mere) parasites... à contrario, in the past, I had to deworm (naturally) some of my chicken several times!!
The same goes for the weather...
...Did you already see ducks swim in winter, despite the snow and the frost?
Duck are hardy, and contrary to chickens, don't even care about the rain... they won't even be too hot in summer if you allow them to swim, while chicken can easily DIE from the heat...!
Regarding duck meat, I don't know what to tell you...
Seemingly, duck meat is good for health, and its fat is healthy...?
(Ducks are hard to pluck, no...? People tell me that...)
But what I CAN tell you : you can have them with chickens, yes.
Just... think about monitoring your drakes if you let them with the chickens : you don't want them to rape your poor girls - hurting or KILLING them in the process.
If you don't buy a good enough number of ducks, you will surely need to separate the drakes from the chickens - especially in mating season (spring and summer).
I know last mating season, I did need to : one of my drakes has continuously tried to rape my chickens until I put him in another park! (He doesn't try anymore to breed with them now, because we are no more in mating season.)
Whatever you choose... good luck!