ats-framework | Axway Automated Testing System Framework | Testing library

thousands of requests per day

that's ... not really a lot, so most approaches will usually be fine

What is the advantage of using JSON over binary here, when bandwidth is a much bigger deal than performance?

JSON wouldn't usually have an advantage; usually that would go to binary protocols - things like protobuf; however, compression may be more significant than choice of protocol. If you want meaningful answers, however, the only way to get that is to test it with your data.

If bandwidth is your key concern, I'd go with protobuf, adding compression on top if you have a lot of text data in your content (text compresses beautifully, and protobuf simply uses UTF8, so it is still available for compression).

it says that the JSON data size is smaller than the actual binary data?

JSON contains textual field names, punctuation (", :, ,), etc - and all values are textual rather than primitive; JSON will be larger than good binary serializers. The article, however, compares to BinaryFormatter; BinaryFormatter does not qualify as a good binary serializer IMO. It is easy to use and works without needing to do anything. If you compare against something like protobuf: protobuf will beat JSON every time. And sure enough, if we look at the table: JSON is 102 or 86 bytes (depending on the serializer); protobuf-net is 62 bytes, MsgPack is 61, BinaryFormatter is 669. Do not conflate "a binary serializer" with BinaryFormatter. I blame the article for this error, not you. MsgPack and Protocol Buffers (protobuf-net) are binary serializers, and they come out in the lead.

(disclosure: I'm the author of protobuf-net)