JSon or XML
The format, JSon and XML represents almost same structure of values. But recently, most of jQuery application use Json.
I think it is because JSon is easy to write, and the format is simpler than XML. When I implement functions handling SOAP and AJAX in Alinous-Core, which is programming language that I made, I felt JSon is light weight and easy to deal.
Response speed is very important thing to do SEO, because the number of pages crawlers get effects search engine ranking.
But XML has better point than JSon. For example, for writing complex software configuration, I prefer XML. Because XML can divide files, and the main XML can includes the child XML files.
The result of converting into DOM variable is same, just the format differ.