1075 vs. O-1

[Cliff Stamp]

I suppose I could ask Roger which he's the most happy with, also.

That would make more of a difference than O1 vs 1074, it isn't just the hardness either you need to know how it is achieved. I would ask the maker about the flex points and impact resistance of the two steels with his heat treatment.

... at high hardness (over ~60RC). If you want 1075 run that hard, you are basically running it as quenched.

You only need about 0.6% carbon to get full martensite hardness of about 66 HRC (assuming nothing else in the steel). Plain carbon steels start forming pearlite within 1-2 seconds at temperature so the big issue is getting them to the martensite start temperature fast. The other issue is that if you under temper them to drop the hardness down in the 56/58 range you are in the embrittlement region because this temperature (about 500F) is where cementite will precipitate along the retained austenite grain boundries and the toughness takes a nose dive. You need to check the impact toughness and strength/ductility curves to find the maximum points. Hopefully the maker has done this, ask them for the data.

-Cliff