My requirement is just to display a set of values retrieved from database on a spread. I am using jquery.
Considering your specific case where you're already doing javascript on the client side, I'd go with JSON for these reasons:
Since JSON is native to javascript
you'd have to write less code on the
client side - Just eval() (or, better yet, JSON.parse()) the JSON
string and get an object you can
use.
At the same time evaluating JSON on the client-side will be more efficient, and therefore faster.
JSON serialization produces shorter strings than XML. Using JSON will reduce the amount of data running across the wire and improve performance in that respect.
Here's some further reading: http://www.subbu.org/blog/2006/08/json-vs-xml