They are essentially identical.
M390, 204P, 20CV.
I would reccomend 61.
But that's the only experience I have working with it. Great edge holding pretty decent toughness. (Heat treat by Peters)
I know you can get 61-63. And I think it may be possible for 64HRC I believe
@DeadboxHero would know more on that particular aspect than myself.
My personal opinion is anything less than 60 is unnecessary for this steel. Its definitely tough enough to support about 60HRC. And with a great heat treat it will perform awesome. Not always about how hard it is. It's a knife not a prybar. Made to CUT.
Take D2/PSF27.
You can get if from some companies (some chinese companies) that boost the HRC to like 62. But the edge holding is crap or it doesnt perform well.
Why? Bad heat treating protocol. Or poor sharpening.
Conversely, you can get some D2 from a good company say Carothers, and have AWESOME results. Why? Perfect heat treat. Dialed in for the steel, and will smoke a poor heat treat.
Seems like everyone is just going on and on about the hardness. It has more to do with the process of how it gets to the desired hardness.