I am starting to research the best carseats to add for when my son outgrows his infant carrier and moves to a convertible. My daughter is currently in a Britax Advocate FF and will be between 2.5 - 3 yo when we need to move my son from his infant carrier. Would you recommend keeping my daughter in a convertible until she is older or is a harnessed booster just as good/safe? I've looked at either the Frontier or Nautilis for harnessed boosters. Our vehicles are a Toyota 4runner and a Toyota Corolla.