They are both great steels in my opinion. Benchmade's ATS-34 holds an edge for a looooong time, but does tend to be a little bit brittle, and will break or chip if lateral forces are applied to small cross-sectional areas. In simple terms, if you pry with the fine tip of an ATS-34 710, it will snap off without too much resistance. Similarly, if you sharpen your edge finer than about 20 degrees per side and you attempt to cut hard materials like metal, the edge may chip. The good news is that it is impressively wear resistant for a stain resistant steel, and the grain structure is such that it cuts aggressively on wood and harder materials.
Benchmade M-2 on the other hand is very tough. It will tolerate a surprising amount of abuse before sustaining damage. It also holds an edge an amazingly long time. It is finer grained than the ATS-34 blade, and will take a very fine, razor-like edge which will tend to perform better on fibrous materials like rope. It is not particularly prone to rust at the edge. My experience is that, even in rainy conditions, a very moderate amount of effort to keep the edge dry will keep active rust away. The BT2 coating does a good job of inhibiting wholesale rusting of the blade.
I don't consider M-2 any harder to sharpen than ATS-34. They are both very wear resistant. M-2 may tend to dull less due to its toughness which means if you used an ATS blade and a M-2 blade for the same job then sharpened them both, they would both require about the same amount of effort to sharpen. ATS-34 has a much greater tendency to form a burr, and polishing the burr off takes some extra strokes. With M-2 though, it will take such a sharp edge, I find myself sharpening it finer just because I know what a scary edge it will take with a little more work. So it is six of one, half dozen of the other.
I personally prefer the fine edge taking ability and the toughness of M-2, but at one point I thought that the toothier edge characteristics of ATS-34 were preferable, since I don't tend to pry or cut metal objects with my knives, and edge chipping has never been a problem for me with my Benchmade knives.
Whew, that was a buffer full. Hope its useful.