I figured the answer was going to be something related to CONVERT having the style parameter (and hence more capabilities), but then saw in BOL (2005) that "CAST and CONVERT provide similar functionality" so chose the last answer.. doh! Should have gone with gut feeling but have been tricked too many times from previous questions which always give the BOL answer as the correct one (even if it isn't). Although I see now that BOL for 2008 does not have that statement in it.

Just can't win....