As in subject, I think there should be no difference between the two tags.
Browsers (tested latest Chrome, Firefox, Safari) support this pretty well already - subtree is not fixed. E.g. this is not touched and rendered correctly as specified:
<button><p>Label</p></button>
HTML should really try to keep up and be more flexible while still being unambiguous. Sometimes it is really hard to understand why certain limitations are still imposed. For example I often wonder why I cannot wrap tag structures with arbitrary tags like <ul><div><li>item</li></div></ul> like I would do with microdata and with arbitrary HTML.
Thank you in advance.