You can treat more complex problems in adults and children. Essentially with the way things are going now, Family Practitioners (in moderate-large areas) don't see patients in the hospital. Most have signed over inpatients to 'hospitalists'.
An office based meds/peds would function very similarly to a FP, with the exception that they could not do OB. Supposedly they would be more comfortable managing more complex problems, whether or not this is true is easy to decide: When was the last time you saw a FP refer anyone to an internist or pediatrician? They usually go right up to specialist: Neurologist, Gastroent, Cardio, etc.
Obviously in the hospital the meds/peds has greater options with regard to patient population.
I would say that if you want to be a generalist able to treat anybody, but think there is even a SLIGHT chance that you may want to do a specialty fellowship, go meds/peds. If you are dead set on practicing in a real rural place and/or know that you want to practice family medicine forever, go family practice. (FP's may 'know'less, but are trained to treat a broader range of things such as OB, and how the family unit affects health.)
