It depends but it can be argued you have no use for either. XHTML1.1 is XML and must be served as XML, but I doubt you have your server set up to serve XML web pages. XHTML1.0 must be served as application/xhtml+xml to take advantage of xhtml but, again, I doubt that you do.
But, even if you did serve xhtml properly, only modern browsers can work with XHTML. Internet Explorer does not. So until IE understands xhtml (IE8 will not) or it goes away (wishful thinking) serving xhtml may be rather useless.
So XHTML winds up being served as text/html and interpreted by the browser as broken HTML, "tag soup". Most browsers handle that pretty well but, like I said, it can be argued there is no point in doing so.