NAIS seems more stable and established … and the fact NA is operated internationally it seems to have a better system in place.. and mobility of teachers within their group of schools.
Malvern will be fairly new but other than that I don't see new staff as unusual or a negative in international schools. I know a couple of the teachers at Malvern who are quite experienced, just new to Malvern but not Hong Kong.
Malvern is a typical franchise international school. Take a well known successful UK private school name and has well-established IB curriculum.