The bio major will have more courses that are similar to medical coursework (anatomy, physiology, microbiology, immunology, histology, virology, cell biology, etc.). In addition, the biology major typically requires you to complete the necessary chemistry prereqs for med school anyway (1 year of gen chem, 1 year of organic, and sometimes biochem). The advantage of the chem major is that it's not just straight memorization like many bio courses. There are courses in chem that may require more analytical skills. I once saw a comparison of MCAT scores of students in different majors. Chemistry majors tend to perform better than biology majors, if I remember correctly.
It's a toss up. Choose whichever major you think you would enjoy more.