Hey Birch,
I wanted to share my experience with starting a small flock with just a couple of each, which I did early Thursday morning with a call from the post office! I've never "raised" chickens or ducks before, although I've been around both. Which is why we decided to start small.
We got 3 ducks and a few chickens (they rode with that bad storm and most of our chicks didn't make it, last lone hardy survivor passed this morning). But I believe that was isolated bad timing.
The differences in chicken and duck behaviors are HUGE! With ducks, in the first couple hours you have to make sure they don't water log by rotating their water 15-on 15-off and ducks *have* to have water with their food. Those were the two biggest out of the box differences. (Also ensuring that the food is compatible and adequate for both.) I had to learn two different "languages" right off the bat. My ducks went through water, and produced poop, like crazy. The chickens...well, they aren't ducks...but I found myself second-guessing trying to take care of two sets of babies at once. Warmth was another issue. My ducks didn't need it quite so warm, which was easy to fix, but again I found myself second-guessing if I was doing the right thing.
Anyway, my point is just this... If I had it to do over again, I would do one species at a time. Enjoy those critters, meet their needs, learn their language. Then, I'd get the other species. And I really agree with the other posters who talked about trying to buy locally.
Good luck in your new endeavor! =)