Both Duet and Aviva fit babies from 5-15 kgs and have great absorbency lasting for 3-4 hours during daytime and 8-9 hours during night time.
1. The difference is that Duet diapers keep your baby's bottom dry since they are lined with microfleece, whereas Aviva diapers will let your baby feel wetness since it is lined with organic cotton.
2. Duet is easier to clean since poop doesn't stick to microfleece, Aviva may need slightly more effort to keep clean.
3. Aviva is much trimmer than Duet.
Using an additional fleece liner with Aviva will help with points 1 and 2 though, especially important for night time.
Your final decision would depend on a) whether you want to use just organic cotton for environmental reasons b) whether the baby is sensitive to wetness and whether you are comfortable using an additional fleece liner to keep the baby dry c) weight of the baby - if closer to the beginning range of 5 kgs, Aviva will be much trimmer.