it is hard to answer the question. there are clearly fewer students interested in health economics (internationally) than ones interested in development economics. there are even a lot fewer faculty members who research these areas. at the schools that produce the most high quality research in this field (harvard, berkeley, princeton and yale in my opinion), i'd say to get to work with any of these faculty members (miguel, gertler, paxson, case, deaton, kremer, canning, bloom, hsiao, udry) is just as competitive as doing development economics there. it is hard to generalize without knowing much about your background. the kind of research that these folks do in international health economics is sometimes intersection between demography and economics, sometimes between macroeconomics and epidemiology and more recently microeconomic models of behavior applied to health.
what are your specific interests in this field?