Axles - chromoly vs mild-steel PKT axles?

Looking to replace a 50mm axle.

I can go with OEM which is ~$300 (Top Kart) or any number of aftermarket such as PKT, Righetti, Swift. PKT seems to come up often as well respected, so I looked into that. They sell “D” variants, which are apparently “mild steel” tubing as well as a “chromoly” variant. Is one better than the other? I’m guessing I should be buying the -19 non-D variant for a medium stiffness axle?

Finally, I was thinking to go with PKT, but are Swift and/or Righetti just as good options? i.e. as long as I avoid garbage brands, are all of these OK and I should just buy the cheapest one?

I’ve used PKT chromoly axles and like them. They used to advertise that they were the straightest axles available. Their shop in Wixom, Michigan is clean and has good equipment for both prototyping and production.