Well, this thread should be in general discussion, as the Nimravus is pretty darn far from a traditional fixed blade. No doubt some helpful mod will move it soon...
The blade steel is marked on the blade.
BM does a wonderful job cryo treating their 154CM - I like it better than some S30V knives I have. I opted for a 154CM blade for myself. It rains a lot up here and I spend a lot of time lying in the mud, so rust resistence is a factor.
M2 steel is very wear resistent and can take a very thin edge. It has a tendancy to rust a lot, but it is a coated blade so I can't imagine that rust is a huge problem. Availibility is an issue with the M2 knives - newgrahamknives had some last time I checked. Finally, M2 will be harder to sharpen.
In the end, beyond a certain point the type of steel doesn't REALLY matter that much. What makes the Nimravus an awesome knife is superior edge geometry, a sharp tapered tip, a thin but tough blade, and a good factory sheath.