I'm in agreement with c. 154CM and ATS34 are still great steels. The heat treat is what does the job! With all the newer steels coming out, especially the cpm steels, some of yesterdays standards are relegated to the back burner and seen a lot of times as being faulty in some way. It just ain't so! I have several knives with ATS34 that have a correct heat treat and cut all day long and when a sharpening is needed, they sharpen right up with no difficulty. I know they are heat treated right by the characteristics they have. Also bear in mind I'm one of those who often times sharpen my knife every other day or so, although all I do is hit it a few licks on the white stones in my Sharpmaker.
If I was limited to either one of these two steels only I would not consider myself to be unfortunate.
Of course if you are planning on buying another 710 and you already have the "standard" model then by all means go for the M2 steel. I've never used it but from all the POSITIVE reports I've read it must be some great stuff!!