I am finishing my Med-Peds residency this year. I chose it because I really do want to take care of children and adults. I am going to a rural location where they are in need of a physician who can do both adult and peds inpatient care, as well as primary outpatient care. None of the FPs will do peds inpatient care, and there are only 2 other pediatricians at this time (Q2 call would really suck!), so all peds go to St. Louis for now. But next year when I get there we will be starting up a Hospitalist program manned with Med-Peds docs (hospitalist - either IM, Peds or IM/Peds is the wave of the future for hospitals).
Also consider income potential. The average Pediatrician (even a subspecialist) will make much less on average then an IM physician, but IM/Peds makes as much as (or even more than) the IM doc, and takes care of kids. Go figure.