1. will give you a better chance move to business if you find you dont like law firm job that much after a few years
2. one more year and 60K more investment than a JD, two more year and 120K more investment than a MBA
3. have to take LSAT and GMAT both and it takes longer time, and it's harder to get into the same school. Different approach of preparing the application materials as well (although Northwestern has a special three-year program and you might want to check it out)
4. for some firms, they dont like hiring someone who holds a MBA/JD because most MBA/JD will end up moving to company/bank later.