I found a link that compares lower and higher Cr content (below), but it's more complicated than that. I don't think PREN is really relevant because it only applies to austenitic stainless steels, and even then only in ferric chloride. 300 series does generally have better corrosion resistance than 400, but it depends on the environment and loading conditions you're working with, and what failure modes you expect. For example, if you're worried about contamination in a chemical plant, 300 series is probably better, while if you're worried about stress corrosion cracking in a higher stress environment, you'd want to go with 400 series (or super-austenitic, duplex, etc.).
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