In terms of actual application at the high school level, there is no difference between a BA/MD or a BS/MD program. In general, a BA has less required hours than a BS degree, hence many 6/7 year programs only have room for you to get the BA undergraduate degre. Whether you get the BA degree or BS degree on completion of the combined program is more of an institutional degree-granting decision by the university, which is usually approved by the specific college (Arts & Sciences, Engineering, Business, etc.) giving that undergraduate degree or they have an agreement with the med school that combined Bachelor/MD students will have certain undergraduate requirements waived as part of their program.