Chickens are the easiest to raise in my opinion just due to the fact that they are less messy and are somewhat more immune to diseases.
Actually, ducks are more immune to diseases than chickens. Ducks are very hardy birds when it comes to illnesses...
I believe ducks and chickens can get along just fine, most of the time... its always good to make sure they have lots of room... as mentioned, seperate food, ducks are indeed messy birds. Ducks need a lot more water than chickens... The biggest problem I could see happening would occur when the birds reach maturity and it happens upon breeding season... a large rooster or drake could become aggressive... It would not surprise me to see a rooster picking on ducks or a drake trying to mate with the chickens... Overall I don't see a problem, especially if they're raised from babies that way.
Feel free to let us know what you plan on getting, and how many of each, and we can make some suggestions as to how much space is needed and such. Good luck.