I don't think GT5 will get to 10 million copies because PS3 isn't as dominant as PS2 for total sales.
BUT I don't think Halo Reach will get to 10 million either. Halo Reach doesn't look to be a vast improvement over Halo 3 or Halo ODST. Graphically, it looks dated compared to Crysis 2, Medal of Honor, and (1 year old) Killzone 2. Lastly, there is too much competition from likely the best selling game this year COD Black Ops (personally, I think Black Ops is going to be the least impressive FPS this year). The American Midwest Walmart shoppers aren't gonna buy 10 million copies of COD Black Ops AND Halo Reach during the Christmas '10 period.
So, I think GT5 will edge out Halo Reach. 9.2 million vs 8.8 million