BlazeDS VS REST +JSON
BlazeDS is going to use AMF for encoding data which is incredibly more compact than JSON. AMF is a binary format so all numeric and date values will be significantly smaller than with JSON. Also AMF uses string references to save a huge amount of space when sending repetitive text (class/property names). Additionally class definitions are also referenced only sent once per AMF message.
Besides the size, AMF is also significantly faster to process than JSON, definitely on the client side and most likely on the server side as well.
Basically, if you have a choice to use AMF over JSON, then do it. If you have to use an existing service that only supports JSON, then it's ok too, but even then if it's a large application or with significant usage, you're better off switching to AMF even for existing services.