Transitional flavors of XHTML and HTML are deprecated. They were intended only for old user-agents that don't support CSS. See explanation in the DTD.
W3C advises that you should use Strict whenever possible, and these days it's certainly possible.
Transitional version has already been removed in XHTML/1.1 and HTML5.
XHTML/1.0 has exactly the same elements and attributes (semantics) as HTML4. The XHTML/1.0 specification doesn't even specify any elements! For anything else than syntax, it refers to HTML4.
Additionally, you'll be unable to use any feature of XHTML that is not available in HTML (namespaces, XML DOM) if you send documents as text/html, and unfrortunately that is required for compatibility with IE and other HTML-only browsers.
In 2008 the correct choice would be HTML4 Strict:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
but as of 2016, there's only one version of HTML that matters.
<!DOCTYPE html>
