bantam vs standard

it all depends on what you want. If eggs are not a priority bantams might be fine for you -
i've only had standard size birds as I love the look of the bigger birds plus I do want large colored eggs.

I have heard that bantams - especially roos can be mean but then that can be true of any roo.

I have currently have a flock of Welsummers, Easter Eggers ( most look like Ameraucana's but they have green vs. slate colored legs so are EE's ), Black Australorps, Barred Rocks, RIRs, White Leghorns and Buff Orpingtons.

My favorite breed is the Black Australorp because of their mild manered but not push overs either
personalities as well as the most beautiful 'face' of the chicken world - IMHO. Their eggs are also
a very nice creamy tan and are extra large and they lay prolifically all year long - mine have continued to lay all winter w/o added light in their coop.

Second favorite breed I have had is the Welsummer as they are so beautiful to look at and the
color of their extra large eggs is magnificent - dark terra cotta colored - some with dark speckled
pattern and many of my Wellies lay eggs as dark as a Copper Marans! They are not as people
tolerant as are my Australorps. Two of my four young roos attacked me so I had to sell them
as I breed for temerament as well as egg production and and SOP characteristics. The Wellie pair in my Avatar photo are a breeding pair I raised from day one from a local breeder.

I'm sure you will hear from many who keep bantams who will chime in as well.

 

Bantams eat less feed and don't require as much space. Mine lay very well (and I have Silkies, which aren't known for their awesome egg production). Bantams, however, are easier prey for predators and will be much easier for your ducks to clobber.
If you aren't worried about showing, keeping the birds clean isn't going to be a big deal. Chickens bathe in the dirt, hence they get dirty
Alot of people complain that they have problems keeping feathered legs clean (like on Brahmas,.Cochins and Silkies). Mine keep themselves pretty well groomed. If your pens tend to stay muddy, a layer if sand will remedy that. You may want to reconsider keeping the ducks and chickens together though (if that was your intention, you didn't really say in your post). Ducks are messy (wet) and chickens do better if kept dry. Many people keep them together, but it seems the dryness is an issue.
Also, if your planning on getting a drake at some point, depending on its personality, he could pose a serious threat to your chickens if he happens to find them attractive. Ducks and chickens are quite different anatomically. Ducks aren't made to mate with chickens, and chickens aren't made to mate with ducks. But, that doesn't necessarily stop an over amorous drake. If seen a few posts about drakes mounting chicken hens and the hens ending up dead because of it. Just one more thing to think about while researching. Good luck!
Nikki