I would imagine that a competitive MCAT score is the one that is also competitive for a MD/DO program. Since the MCAT score is more towards the MD that you plan to pursue, should not the score be as competitive/as high for that? Also, some programs will accept the MCAT in lieu of the GRE under certain conditions. If you are enrolled in a MD/DO program and plan to take "a year off" to pursue the MPH, programs will more likely welcome the MCAT score. However, if you plan to do the MD/DO after the MPH, it is less likely (though no impossible) that a program will accept the score in lieu of the GRE, since they cannot be assured that you will pursue the MD/DO thereafter - and therefore, the MCAT cannot presume to reflect your (testing) abilities. My recommendation is to shoot for the score that not only best reflects your overall capabilities but also best fits the MD/DO program to which you hope to gain admission.