Everyone I have talked to has told me that IM/Peds is more competitive than IM, i.e. in terms of getting a residency position and the residency itself. I think you have to show a passion for both peds and IM, which can be difficult. Also, some IM/Peds residencies you go back to being an IM intern after two years of Peds so I think that generally IM/Peds residents are supposed to be harder working than categorical IM residents and are expected to know more too, even though they have to master both fields.