You should use XHTML complaint code ( ie <br />, <img src="crap.jpg" /> ) if you'd like to maintain compatible with future browsers, and if you'd like to be nice to whoever is going to have to maintain the pages you build.
Otherwise, use regular HTML ( <br> ).
But for the record, XHTML is basically a stricter standard for what HTML now. XHTML works to solve all the crap problems that should never have popped up in HTML to begin with.
Basically the only major diff. between XHTML and HTML ( as I understand it, correct me if I"m wrong ) are the following:
Every open tag has to have an ending tag OR end in /> ( hence the <br />, but you could also do <br></br> )
All elements of a tag must be surround in quotes ( ie not more <img height=100 src=crap.jpg>do <img height="100" src="crap.php" /> inestead )
Must maintain good nesting practices ( ie this is bad nesting: <tr><td>stuff</tr></td> )