The Forza games have quality wise been right up there with GT, not as good as GT but will leave that for another day. At least in NA I expect Forza 3 to beat GT5 in terms of sales but in Japan and EU GT5 should wipe the floor with Forza 3.
I would think GT5 will do 1 million week one in NA alone however think Forza 3 should do 1 million WW first week.