It's really hard to say which one of these steels is better, even if you know what characteristics you want to prioritize. Steels are basically a give and take: You make it better in one category, and it gets worse in another. 15V and M4 have about the same corrosion resistance, so it really comes down to whether you prioritize wear resistance or toughness.
According to Larrin Thomas's testing (https://knifesteelnerds.com/2021/10/19/ ... esistance/
) M4 is about twice as tough as 15V, but 15V has about twice s much wear resistance. (Larrin calls it edge retention, but I think wear resistance is a more self-explanatory descriptor).
That article also goes into the merits of toughness vs edge retention, so I won't unnecessarily parrot it, but in a nutshell a less tough steel is more likely to chip or fracture. So for example, if you do a lot of prying with the tip of your blade, or if you do a lot of twisting/torquing types of cuts that will exert lateral pressure on the edge, then you might want to prioritize toughness over wear resistance.
On the other hand, two caveats about Larrin's data: He ran his M4 at 61.1 HRC and his 15V at 63 HRC. However, Spyderco runs both the Shawman M4 and the Manix2 15V at about 64-65 HRC. So consider both of the toughness ratings to be less than what is listed, however
with a higher hardness there is also more edge "stability" or "strength". In other words, a soft steel will get dented more easily than a hard steel, even if it's "tougher". The trade-off there is that while it will resist plastic deformation like rolling, denting and bending, non-plastic deformation like chipping, cracking and fracture become more likely. The other caveat is that Spyderco is using Shawn Houston's special heat treat, wheres I think Larrin was using the protocol specified by Crucible. You can read more about Shaw's goals here: viewtopic.php?f=2&t=82056
But to sum it up, he was really striving to find something with edge retention as close to Maxamet, but with toughness closer to K390/10V.
But, another little wrench in this gear... The blade geometries are so different, that it's not really fair to compare them based on the numbers. Wear resistance and how long a knife can keep cutting is greatly affected by the blade thickness and geometry, as well as its resistance to non-plastic deformation.
to tl;dr all of this... All I can give is an opinion, that the Shaman's M4 is going to be a better steel for you if you want something to stand up to tasks like, prying, or twisting cuts that impart a lot of lateral stresses on the blade edge; but it's going to suffer even more in terms of edge retention and cutting ability because of its geometry. On the other hand, the Manix2's 15V is going to be better for you if you you are keeping it strictly to linear (up/down) cuts and slicing; but it's going to be more prone to non-plastic deformation if you subject it to things like prying and twisting cuts. Either one should have pretty decent edge stability and resistance to plastic deformation.
All those things considered, some more practical advice: If you were looking to get a Manix2 in 15V for a decent price, that ship has sailed. They're really only available on the secondary market now for hugely inflated prices. But if you're willing to pay $300-$350, I would say get it while the getting is good. (I would also really strongly discourage you from helping the scalpers from making a profit, but that's a personal politic, so take that or leave that).
Beyond that, if you were leaning towards the Manix2 only for the 15V, there are a lot more options coming out in 15V soon, including a Shaman. I believe announced so far are the ParaMilitary 2, the Shaman, and the 15V Mule. But there is also going to be a collaboration folder between Shawn Houston and Spyderco someday soon.
Meanwhile, the CPM M4 Shaman seems a lot more accessible and for a fairer price and since it wasn't a sprint run, it's more likely to stay available on the secondary market for better prices than the sprint runs, which are pretty limited in pieces.
In my opinion, as someone who has a 15V Manix2 and loves it, and has never used a Shaman, I would honestly actually recommend the Shaman simply because I think there's less chance of disappointment. If you end up paying a huge inflated price for the Manix2, there are some odds you won't even like it. Plus, it will help you get a feel for if you even need or want a steel with more wear resistance.