The difference is the additional back support for Synergy. Cause once your little one hits more than 7kg, you will feel the strain. The additional back support from Synergy aids in lightening the overall weight, ensuring the weight is disperse evenly.
If you are looking at something of shorter term period, the Air will works fine, but if you are looking at something that serves you till the baby hits more than 10kg. You may want to consider getting Synergy instead.