One way to compare FP and Med/Peds is what you can do with one degree but not the other, i.e.
Family Practice: You have the possibility via extended training in ob/gyn during residency to deliver babies in certain controlled settings, i.e. some FP programs have a big focus toward ob/gyn. Or you could do minor surgeries, some FP programs have a focus in minor surgeries. You could go on to get one of the few fellowships offered to Family Practice residency graduates like sports medicine, geriatrics or adolescent medicine and that is all!
Med/Peds: You are board certified in both peds and medicine, and therefore can receive a fellowship training in any area offered to pediatrics or internal medicine residency program graduates i.e. cardiology, pulmonology/ICU, critical care, hematology/oncology, infectious disease, nephrology, GI. If you do do a fellowship, let's say in hematology, then you could see a patient with sickle cell disease when he is 1 year old, 5 years old, 18 years old, 35 years old. You basically have more fellowship options, but you can't deliver babies, and you can't do minor surgeries.