Good match-up, these two are near equal and are opposites as well, they cancel each other out. But I think Ace'll wins, he can just generate bigger blasts of fire, quicker and with less apparant effort than Hitsugaya.
And It'll take waaay to long for Hitsu to get enough ice to stop a Entei http://www.mangafox.com/manga/one_piece/v46/c441/16.html.
Though Hitsu might have the edge physically, so that may even it out.