Should I priortize one over the other?
generally speaking, no. a good balance of both is required for optimal survivability. certain champions can favour one over the other, particularly if their damage scales off one or the other, but even then both are important. hp will have more of an impact on stage 21 since the boss ignores defense, but as skadi mentioned he won't be all that useful there unless you have a melga or maulie to bring him back as well.