The fiberglass batts would probably be the easiest to install. The only problem will be the depth of the cavity that you are going to be putting the insulation into. My husband actually added an extra 2" board to the 6" rafters when he insulated the ceiling - so he could get a higher...
Make sure you have enough ventilation to keep your birds healthy in that shed. Excess moisture and ammonia build-up will be more harmful than the cold.
Put the ventilation holes in the coop before you insulate. I would frame out some ventilation openings above the highest cages that you will...