The training is very different...IM focuses on comprehensive adult medicine...so besides a general outpatient clinic you rotate through cardiology, pulmonary, oncology, etc. Your understanding of adult medicine is thus far more nuanced than FM. FM includes inpatient gen med type rotations, general peds, ob/gyn, and a number of different outpatient clinics. So, in essence they're trained to provide primary care for the "entire family," something an internist is not trained to do.
As attendings one can tailor their practice so much of a difference isn't apparent. For example, a FP physician who doesn't do obstetrics (more common than not) and focuses more on adults is really doing the same thing as a primary care internist. I would think that such a physician would probably have been better served doing an IM residency because of the more rigorous training in adult medicine. But if you wanna treat kids and/or do ob and definitely want to do primary care, FP is the necessary training.
As alluded to in earlier posts, an internist can choose to work as a hospitalist doing solely inpatient adult medicine or complete a fellowship in cards, endocrine, GI, onc, etc. Its important to realize that the skills learned during internal medicine training are required as a subspecialist as well, even in procedural fields like cards and GI. There are FM fellowships like sports med but in general they are not as commonly done as the IM fellowships...most people who go into FM plan to do primary care, which is not true of at least 50% of those going into IM.