This was mentioned by Nicholas Zakas of Yahoo in his artical/talk at Velocity 2010,
it sounds like you're into javascript performance so it's well worth checking out the slides/pdfs.
includes stuff by Steve Sounders and a load of people i've never heard of:
http://en.oreilly.com/velocity2010
edit: if i rememer correctly the conclusion was html is usually better due to IEs slow json parsing (i think!)
