The thing is, Forza is like the child's version of GT. When you're a kid you play Forza...when you grow up you play GT. Yeah Forza has tuning options and all that, but GT goes even further into tuning. I think Forza is more of a showboating experience. They put a lot of emphasis on livery and the way your car looks, GT is more about tuning expertise and finding that perfect setup.
Even without all that, the feel of the cars in Forza doesn't compare to that of GT. That being said, GT isn't for everyone. I think if you aren't nuts about every aspect of cars, then you'll find GT frustrating and boring. Forza makes it easy for people who don't understand suspension dynamics and what not, you can just go online and buy people's tuning setups with in-game money, and not even bother with having to do the tuning yourself. GT is for the hardcore people.
Even GT's creator, Kazunori Yamauchi raced a Lexus IS-F in the 24 hours of Nurburgring (you can see the videos on GT Channel, and on the Gran Turismo website)! With a company such as Polyphony (who also designed the graphical displays for the new GTR) so involved in motorsports, you can't expect that they'd produce anything less than the absolute BEST racing simulator available.
