I've used S-12s, M-12s, 756's and 755's. IMO, the S-12 and the 756 both wear much better than average and have exceptional performance in soft and intermediate terrain.
The M-12 is among the best wearing knobbies ever, but it does not perform as well in soft/intermediate terrain as the S-12 or 756. For summer riding or dual sport, the M-12 wears very well, especially the rear. Due to the thick sidewall, you can run low pressure on the M-12.
The 755 will chunk in the rocks or dual sporting, even the front. In soft terrain and when fresh, it will match the 756 or S-12, but does not wear as well, IMO.
To summarize, either the S-12 or 756 are excellent tires for any conditions except hard-pack.