Hay is mown grass, generally used as feed for livestock. It therefore has a certain value to farmers or horse people. Straw is used as bedding for livestock, but is generally cheap, and available in quantity. Also, hay usually has a higher moisture level than straw due to differences in harvesting methods. Hay is also more prone to overheating when compressed.
Straw is inexpensive, renewable, long lasting if kept dry, and has good insulating value.
Paraphrased from February 25th video conference. Paraphrased from February 25th video conference. We used 40 bales for a wall 10 feet by 13 feet. - Paraphrased from February 25th video conference. It took 2 or 3 months to plan and 2 days to build. It was a typical construction project where you have to solve little problems when they arise.