The CarryOn is designed for larger children, as it features a weight range of 20-60 lb. It has a roomier panel and more adjustability in the straps to get a comfortable fit for children in 2T pants or larger. Children can be worn on the hip, the back or facing in, in 3 positions.
The COMPLETE weight range is from 7-45 lb with no need for an infant insert, so this is the proper option for children under 20 pounds. The COMPLETE has a fold down neck support so smaller babies can be in sight at all times and infants 6 months and older can be worn facing forward. Children can be worn facing in, out, on the hip, and on the back in a variety of ways because the width of the seat adjusts.
Both options are terrific for the toddler years, so you'll want to consider what you need in terms of weight range and features.
Check out this great blog post for more information on when to make the switch to our toddler carrier: http://lillebaby.com/blog/lille-lessons-when-are-you-ready-for-the-carryon-toddler-carrier/