Volume and clinical capabilities are more important than the additional presence of adult services. There is a positive correlation between the former and freestanding children's hospitals, but I'm sure there are university centers with higher volumes and more developed specialty services than some of the smaller freestanding hospitals. It depends on your career goals as well- for outpatient and related subspecialties, likely less important than if you were planning on being a hospitalist. Of course, the proliferation of fellowships (including hospitalist) makes residency location less important, as it is unlikely you would do a fellowship not at either a large academic or a freestanding children's hospital. But if you're interested in, for example, PICU or NICU and all of the sickest kids get shipped from your center to the bigger places in town, it may deprive you from that educational exposure.