It isn’t which steel is better, it is which steel will be best suited to a particular application. Air hardening steels harden in the air and are not subjected to the sudden shock of quenching. They are well suited to applications such as stamping dies where dimensional stability is important. Water hardening steels are used for sissors and knives and cost less than the other steels. This is because the mix of ingreadents to make the steel uses less exotic and costly materials than the other steels. S7 is used for high shock applications but will not wear as well as the others. Oil hardining steels are used for cutting tools such as end mill cutters. P series metals contain a high chromium content and can take an extremely high polish. They are used for injection mold cores and cavities. Some steels cost a lot more than other steels but it does not mean that they are better just because they cost more.