In the genetics and biochem classes I took, there was very little overlap between the two. Genetics focuses on gene mapping, inheritance, vectors, recombination, and all that jazz. Biochem focuses on (as the name implies) biochemical pathways. You'll see a ton of cycles (Krebs, Calvin, etc), photosynthesis, and discussions on enzymes, allostery, and amino acid specificity. I would go into further detail, but God knows I forgot just about everything in those 2 classes 😛
If you had to pick one over the other, I believe biochem will provide better preparation for medical school.