In my experience, the only thing ATS-34 beats M-2 at is stainlessness -- ATS-34 is marginally stainless, M-2 is not. On the other hand, Benchmade's M-2 will easily hold an edge as long as ATS-34 (longer, I think), and is much tougher, so you can put on a thinner higher-performance edge without worry about chipping. I think Steve Harvey's M-2 AFCK review might be on the benchmade site, it's worth a read.
For me, the big downside of M-2 is that from most companies, it only comes with a coating. I dislike black coatings.
Joe