hay

I would say hay is more likely to cause impacted crop because the hens will eat it and the bits can be quite long.

Straw can cause respiratory diseases as it is hollow and mould spores can develop in it.  Red mite can also hide in the hollow.

We use wood shavings as advised years ago by professional poultry keepers.  Also stuff like aubiouse and other bedding generally sold for horses is good although a bit more expensive.