M-2 is a steel designed for superior edge holding at temperature of 1000°C (sorry, the rest of the world uses the metric system). It is mainly used in drills (marked HSS high speed steel). M-2 isn't excactly HSS, HSS is a little lower alloy, but the characteristics are the same.The better edgeholding comes from the fact that M-2 can be hardened way beyond 60 (like ATS-34)and still not be brittle. A-2 is not the same steel. It is totally different. A-2 is for those who require extreme toughness and pretty good edgeholding, but don't care for rust. A-2 rusts faster than M-2, because M-2 is a higher alloy, with more chronium. My preference for a knife-steel would be 52100, differentially tempered. Better edgeholding, a lot tougher and rusts the same. I think all stainless steels are only good when rust prevention is a factor. And I don't care for a little dark spot on my blade.
B.T.W. ATS-34 rusts too, just not so fast. Just take it out camping and cut jour steak with it, and let it lie there till the morning. It will be stained, about as much as the same threated M-2 blade.
Hope this helps.