It'll likely sell 2x the amount lifetime (at least) that Forza 5 will. So for all those complaining how less than stellar GT6 is, it won't really affect the sales.
Btw, GT6 is at an 80 on metacritic, Forza 5 is at an 81. Just pointing that out.
Oh, and I hate both games, and sim racers in general, due to the fact that they suck at being games while exceeding at being sims, which are boring in my opinion.
Have a great day ;)