IM is focused a lot on inpatient medicine plus sub-specialties. FM is technically supposed to be the only "true" primary care specialty with a focus on outpatient medicine. You can definitely do outpatient primary care with internal medicine, or you can be a hospitalist as a family medicine physician. When it comes to that cross-over, you can see some head butting. IM physicians will say they are better equipped to handle more complicated patients, while FM physicians will say they can do the same thing. Then you get a collective circle-jerk with bashing one another. At the end, it's best to work your hardest to do what's best for your patient, and not your ego.
tl;dr: Do IM if you want to practice hospital medicine or go into sub-specialties. Do FM if you want to practice primary care in the outpatient setting.