Yosemite, self closing tags are not valid HTML. It is not a huge problem in practice because most browsers ignore the slash, but it is nice to get pages to validate.
chradil, I want to use HTML 4 because its the best way to get as many browsers as possible into standards mode. See this table.
XHTML strict also looks OK from this point of view, but I am more familiar with HTML.
I also need to use snippets of other code (web stats, ads etc) and these are more likely to be valid HTML than XHTML 1.0 strict. That said most of what I use at the moment does validate as XHTML 1.0 transitional so it SHOULD be OK.
Then there is the whole issue of sending XHTML as text/html, although I do not really understand what the practical implications of that are.