School choice researchers have used a range of measures to capture competition between schools, but few have assessed the relative utility of these different measures. Using district-level transactional data, we analyzed the relationship between nine measures of school competition and inflows and outflows of students to and from Arizona school districts: a) two measures of market density, and measures of market share and market concentration, and b) five measures that combine density, market share, or market concentration with proximity. Our analysis suggests that the number of charter schools within 10 miles of a district's boundaries has the most statistical and practical significance as an indicator of competition.