I have a FAQ about treating respiratory ailments here and it covers the antibiotic choices.
http://www.hencam.com/henblog/respiratory-diseases/
Chickens can get irritated throats from dust and mold. I find that straw is not as absorbent as shavings and can get moldy. Chaff (chopped straw) can be an irritant. You might want to switch to pine shavings. (Here in New England it's affordable.) Also, some hens have respiratory ailments because they are too fat! As much as they like treats, they don't need them. Make sure that the bulk of the diet is from laying hen pellets and greens. Don't feed corn, and the sunflower seeds should be a small handful for the flock.