You will almost always want the behavior in $SUBJECT. In fact, I think that the order of the tabs should probably be changed to: Topic Details | Topic Tags | Topic XML
This is actually because of a bug in the CodeMirror XML textbox styling. It won't display properly if the texbox is not visible when the page is rendered, which means the XML tab needs to be shown first.
We opened another bug after this about merging the "Topic XML" and "Topic Details" tabs together. I think that solves this too.
Fixed in 20110831-1522