That was easy,

Bet IASbet SuperPrice and be guaranteed that the payout will exceed at least 2 Australian TABs, in fact SuperPrice often exceeds all Australian TABs for Win, Place, Quinella, Exacta, Trifecta and Multi-betting.

So probably better IAS if betting Country Best (best of NSW & Supertab) though not necessarily: eg. NSW $1.04, Supertab $1.40c, Unitab $1.10, so you are paid maybe $1.11c, whereas Sportsbet would have paid $1.40.

So it all depends on what has happened over a long period of time, does anyone have archived details on this??