The metric is speed and size of the data.
JSON is faster then XML in terms of speed. It's smaller then XML in terms of size.
XML is bloated to allow you to represent and validate structures.
However there are various BSON formats around where people take JSON and then hand optimise the storage format excessively. (BSON is binary JSON)
Some BSON spec I picked from google
Bison, A JavaScript parser for some arbitary BSON format.
Now if you really have bottlenecks with transferring data (which you probably don't) you may want to use WebSockets to send data over TCP rather then HTTP, thus reducing the amount of traffic and data you send.
Of course you only care about that if you making say X000 requests per second.