When should I choose a family medicine over an internal medicine doctor?

For the whole family. If you have children and want the whole family to receive care in the same office, you might choose a family physician. In some areas, family physicians also do some obstetrics .
Find a good doctor. Physicians in both of these specialties treat adults, but internal medicine doctors almost never treat children or pregnant women. For adults, family physicians tend to focus more on prevention and outpatient medicine whereas internists usually do more inpatient (hospital) work. As an adult, i see a family physician for my medical care, but would feel comfortable seeing a good internist as well.