RPG, a few key words from your post:
10 million people are not going to go on gaming forums and see reviews for GT5 but no doubt it is going to be a very highly rated game. The majority are still going to choose GT over Forza, not say "i'm so poor I will have to get Forza 3 because I cannot spend another £100 for a PS3".
Key #1, about 10m people not going on gaming forums to read reviews:
As much as you are correct in saying that not all will, it's inevitable that some will. If GT5 does not ship as a better-reviewed game than Forza 3, then it may see some sales lost to a game for a system that's half the price. Unless you assume that the 10 million GT5 owners are already owning PS3s.
Key #2 'the majority are still going to choose GT5 over Forza'
Key word: Majority. What kind of majority are we talking about? 51%? 70%? Even if 90% take GT5 over FM3 due to price concerns of investing in a PS3, then there would be a few losses of software due to FM3.
Also, if these statements weren't true, why did Grand Theft Auto IV on PS3 exhibit such inferior sales to the PS2 versions? Yes, I know it's multi-plat, but I think that, given VERY similar games, we may see some cross-over for racing enthusiasts.