When used correctly, XHTML is lighter than HTML.
It's suggested to not use table anymore with XHTML for the global display of your site. You should use <div> for blocks(like a cell with HTML), lists(<ul> and <li>) for menus, the headers(h1, h2, h3...) for title and so on...
Of course, XHTML is maybe harder to understand than HTML, but when we know all its advantages, XHTML is better.
Just as an exemple, see at the code of my design here: http://sadike.free.fr/sadike/ftxp/index.html . If you want, try to do the same thing just with HTML and tables.... I'm sure that you couldn't do lighter!
As a web designer, I use XHTML exclusivly at this point. But, a few quick points :
XHTML does not equal DIV / Tableless
HTML does not equal INVALID & TABLES
truthfully, anything that can be accomplished in XHTML can be accomplished with nearly the same code in HTML. the difference is that XHTML is XML, where as HTML is not.
why is this important? Short answer, for many projects, initially, it really isn't. It's a choice, the addition of the extra tags and slashes adds a nominal bit of overhead, if any.
BUT. If you have ever taken over a site from another designer, XHTML is wonderful. Easier to read, because all the nesting MUST be done right for it to be valid.
Also, XHTML has the possibility of adding your own tags, with a custom doctype (so does HTML really, but it's more accepted to do this in XHTML). things like <newsitem> or <red> could be used (before the semantics nazis sic the dogs on me, note that was a 'could' not 'should')
All that said, XHTML is going to be what people are moving towards, it is the so called 'furture'.
One thing I havn't mentioned is the application/xhtml+xml thing. Note that as of XHTML 1 (not 1.1), XHTML documents SHOULD be served using this, not MUST. This dosn't happen until XHTML 1.1 or XHTML 2 (not yet a full standard), and there are a number of PHP solutions to do just this.
I'd be more than happy to answer any specific questions about XHTML vs. HTML as I know em, just keep the followups coming
~just one web developers opinion - j