I have used both, and I prefer Straw. That said there is a big difference between hay and straw. Hay tends to get and stay wet, and is very prone to get moldy. I also add Stall Dry to the straw in the bottom of the coop. ST helps dry out the poop and what have you in the coop. I have not had any problems with the straw getting moldy. I also like the insulating qualities of straw. I do use wood shavings for the brooder, it seems easier for the chicks to run around in.