What is the difference between Gran Turismo and Forza?

[Torillian]

I believe that the Forza series allows for more tuning of your cars to a significant degree while GT series has alot more believability in in the sounds of the cars and also has some deals with licenses that Forza does not such as Nascar.

Haven't played either, don't like the genre, but those are the differences I have picked up on.