Better is always dependent on the person, so no one can say which is better for you. But I can provide some info.
Although I know nothing about Mayo except that is a great hospital in general (dont know anything about the pediatrics), I can discuss SLU.
I went to SLU for medical school and my experience at Cardinal Glennon is what drove me into pediatrics. It is truely a wonderful place. The faculty is outstanding. They really care about the residents and do a great job teaching. The patient care they provide is the best I have seen among all the hospitals I have spent time in since starting medical school. The general peds training there is excellent. Traditionally, research has been lacking but the chairperson is focused on further expanding the academic standing of the hospital. They have a fairly newly expanded hospital with 2 new inpatient floors, ER, and outpatient center with renovations to the older part of the hospital. They are currently building a new center for the NICU, PICU, and OR. They just hired a new cards chair and a few heme-onc attendings with plans to further expand several of their subspecialties. It is often overlooked by applicants since it is in the midwest and largely overshadowed by Wash U. I think it is largely underrated and it deserves a very close look during your residency search. Good luck!
