Dan, both theory and personal experience indicate that you should get better overall sound quality with the vertical M2. The better dispersion should benefit both off-center listeners and those near the center, since the off-axis reflected sound is significant even in the center.
Specifically as to dialog intelligibility, however, there may be little or no difference on those films which are causing trouble. As Alan and others have pointed out and Jason mentions now, the problem is that some films have the dialog mixed too low for easy intelligibility. The main problem is sound quantity, not sound quality. It's often suggested to set the center speaker 3-4dB higher for such material.
