The CarryOn is designed for larger children, as it goes from 20 up to 60 pounds. It has a roomier panel, and more adjustability in the straps to get a comfortable fit. Children can be worn on the hip, the back or facing in, in 3 positions.
The COMPLETE has a fold down neck support so smaller babies can look out and be worn facing forward. Children can be worn facing in, and out, on the hip and on the back in a variety of ways because the width of the seat adjusts.
The COMPLETE weight range is from 7 pounds at birth to 45 pounds with no need for an infant insert, you cannot use the CarryOn with children under 20 pounds.
Check out this great blog post! http://lillebaby.com/blog/lille-lessons-when-are-you-ready-for-the-carryon-toddler-carrier/