In general, in theory, browsers eat everything. BUT: syntactically (xhtml strict) are valid 1 and 2, and only 1 will work everywhere (some older browsers consider the slash to be part of the tag name).
If you get used to write all the single tags in the form
<tag />
, there will be no problems with either the html versions (for validly everywhere), neither with cross-browser compatibility, nor with xml (for there will be no habit of leaving open tags).