Molecular biology and Genetics are NOT the same thing. While they can be similar, Genetics is focused more on the study of inheritance. It is diverged into two subtopics--molecular genetics and classical genetics. Molecular genetics deal with Genetics at the DNA or Molecular level. You will study the process of transcription and translation as it pertains to inheritance, DNA replication and control of gene expression. It may also discuss mutations that arise as a result of damages, mistakes etc. The other road of genetics is classical genetics which deals with mostly Mendelian inheritance, studying how genes are inherited (from parent to offspring). This is where you learn about dominant/recessive genes, crossing and punnet squares, pedigree analysis of disease, and so on. At my college, an introductory upper division genetics course integrated both molecular genetics as well as classical genetics (also known as Mendelian genetics).
Molecular biology is very similar to Molecular genetics and sometimes can be used interchangeably. If you take a course in Molecular biology you will almost certainly not cover the topic of classical genetics (or Mendelian genetics).
IMO, I would suggest that you take Genetics as prepares you better for medical school.
Also be advices that taking upper division biology to prepare for the MCAT is not the best decision.
hope that helps