M2 holds an edge longer and requires more of a beating for the edge to roll or chip. Because its higher tougher, you can run your edges thinner than with 154CM.
154CM, though, is more than good enough for most outdoor uses and, since many folks like to grind their edges at a relatively thick angle, it's more than up to the task.
If you're using your Nim to dig, pry, or hammer and try not to let it get too wet, M2 is your pal. It will safely hold a steeper edge than 154CM and has a higher wear resistance, so it has the potential to cut better than 154CM (assuming you go after that narrower angle).
154CM requires less protection from the elements than M2 (though Benchmade's BT2 does wonders to help M2) and is a very good cutter in its own right. 154CM will also sharpen easier in a Spyderco Sharpmaker, if that makes a difference.
M2 is my recommendation.