Hi Gerald - 

The big difference between the 710 MB and the ZM is the "z" muscle design of the ZM club-head.  This design moved more weight from the heel of the club towards the center.  This allowed for a lower, deeper center of gravity compared to the MB design.  The reason Titleist went back to the traditional muscle design found on the MB is due to its feel and playbility.  Many of our Tour players preferred the feel and feedback of the traditional muscle design.  Also, the MB design has more club mass found directly behind the center of the club face.  This allows for more solid feel when hitting "high on the face", which typically happens when playing out of rough.  The sole of the MB has been designed to create better turf interaction over the ZM design.

Both are excellent club designs but the MB is the current preferred Tour design, due to its workability and overall "feedback and feel".