Because internists have an older patient population (normally 16 or so is the youngest), they tend to manage more complex internal pathology such as diabetes and hypertension. They also tend to have more patients who are hospitalized. You can opt to only do ambulatory care as an IM specialist, however. In fact, I believe the only residency programs for IM that are exclusively ambulatory-track are AOA residencies. On the other end of the spectrum is the hospitalist, who is an internist who only sees hospitalized patients.
FPs see the entire spectrum of patients, birth to death. They tend to have more ambulatory cases and see fewer patients in the hospital (unless they are rural practictioners) and their focus is often on prevention and health maintenance rather than disease management.
Of course there are many variations, but this is a general outline of the differences.
You have to weigh the advantages yourself. I would suggest going to the IM and FP residency boards on SDN and ask those folks as they are on the front lines.