DX10 Assassin's Creed Faster Than DX9

PCGH has a quick Q&A with Charles Beauchemin of Ubisoft about how the PC edition of Assassin's Creed will run faster under DirectX 10 than DirectX 9, though both versions will look the same. They discuss why this is the case, though this all applies to Windows Vista, and it is not stated whether Assassin's Creed will run faster under DirectX 10 and Vista than it will under DirectX 9 and Windows XP.