Both are very good schools, with high average SAT/ACT scores, and matriculation to quality colleges like Vanderbilt University, University of Georgia, University of Tennessee, and Duke University, and University of Texas - Austin.
Comparatively, McCallie School is strong in both academics and extracurriculars. It offers a wide variety clubs and organizations for students to get involved in.
One thing to note is that McCallie School is a boys school and the Webb School is co-ed. Generally speaking, a co-ed school is slightly more competitive in admission comparing to a single-sex school, while it also depends on the school.