You may want to forgo both for the moment and go with HTML 4 strict. HTML does not have wide browser support yet, and XHTML is now an abandoned standard.

I'd hold off on HTML 5 & CSS 3 until both have full support. Browsers make cross-browser design hard enough.

I'm for the XHTML, too... But keep one (or even better both) eye on the HTML5. There are still many undefined elements.

I personally go with HTML 5 but avoid using the new HTML 5 elements for now. This way your code will validate and display correctly in all browsers. It also shouldn't be too difficult to upgrade to use more semantically correctly elements when HTML 5 browser support is improved.

Also, why use CSS 3 and not HTML 5? CSS 3 support is even worse.

Well from my point of view i see you should use Xhtml strict with css 3.0 effects , Html 5.0 still IE & alot of Browsers versions didn't support it !
So your web site well looks ugly with some browsers ,
thanks a lot, wish you best

I would (and do) go for HTML 5, as you can even enable HTML 5 for non-compliant browsers with the help of JS.