ATS34 is a damn fine stainless. 154CM will be a very slight improvement, but it's very close to being the same steel.
1095 is good stuff, a high-end steel just below what's considered "tool steel" and with a good heat-treat, the edgeholding is better than ATS34. It'll rust, but Black-T helps with that issue.
Current D2 compositions are a good tool steel with "borderline stainless properties". Only widely reported downside is a rep for being a bit of a pain to sharpen, with reports varying depending on the maker - so it's probably connected to the heat-treat. BUT, REKAT is talking about a modified D2 with even greater rust resistance, so we have no idea how easy that'll be to sharpen.
My thinking is this: REKATs like the Hobbit Warrior and Sifu are "pure combat monsters", not utility knives. So you want a really good edge that'll stay that way, versus "micro-rusting" on the cutting edge and "going dull in the sheath". Under frequent outdoor carry, I'd be willing to bet the 1095 pieces might do just that, especially in a legal climate such as mine (California) where open-carry is required on fixed blades.
So REKAT's switch to what amounts to two grades of high-end stainless steels makes sense for them. ATS34 and it's close kin 154CM have enough stainless properties, edgeholding and toughness for just about any type of personal defense knife. The D2-M will add extreme toughness and edgeholding on top of that. But the HW blade design is such that there's just no way you're gonna break it in a fight even with the "weakest" steel we're discussing here, ATS34.
If you want an ATS34 piece now, and you find one at a good price because the dealers know it'll soon be "obsolescent", go for it. My daily carry piece is the Sifu in ATS34 and it's a damn fine blade...when the D2-M version comes out, I'll probably switch to that as daily but that's mostly because my Sifu is serial number one and really deserves to be kept pristine
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Jim