Look, Both GT and Forza are good games.
It is however very unlikely Forza has any chance to be a GT killer as based on sales of F2 vs GT5:P it's pretty clear that GT5 will very likely sell more than double F3 (i.e. the sales of GT5:P indicate that GT5 is likely to see at least 8 M in sales while sales of F2 indicate at least 3 M if not more than 4M)
2 disks ain't a big problem but it will (if true) give more ammo to Sony's claim that over time more storage for certain games will be required.
Both will very likely offer a fantastic racing experience.
Can we leave it at that?
Try to be reasonable... its easier than you think...