When it comes to learning on or the other, there's really rather little between them. XHTML is essentially a subset of HTML that encourages (or rather requires) stricter standards -- specifically, it's an application of the XML standard to HTML. As such, any valid XHTML is also valid HTML (for the most part at least).
In my opinion, the distinction between XHTML and HTML isn't really that important. What is important, however, is to write consistent and efficient markup, and this is what the XHTML standard was designed to encourage. It doesn't matter whether you label you code as XHTML or HTML, just as long as it's well-written.
The main feature of XHTML is simply that it requires a high standard of quality in your code, but this is something you should be doing anyway in HTML.
