Around mid 2008 they were saying that GT5 was still over like a year and a half away so I doubt it's coming out in 2009. Anyways, I think GT5 will outsell Forza by alot. Their average scores on metacritic are like less than a point from each so I doubt one will be a drastically better game than the other.
Forza 1 sold like 1 million copies and Forza 2 sold 4 million, but I think that's only cuz it was a bundled game for a long time. Meanwhile, the GT5 Demo sells 3 million copies and the lowest selling real deal GT release moved over 9 million units.