GT is a great sim but a lacking video game. Inferior community features, car personalization, and the 800 ps2 cars just made me raise an eyebrow....wondering what they were doing for 5 years.
Forza on the other hand releases only AAA games (90+ on metacritic), and comes out every 2 years.
Frequent releases, maintains AAA quality, more community oriented. its a no brainer to me.
Older and wiser. Still bias and proud though ;)