I'm gonna tell you right off why i dont care about Forza but i do about GT (in very far between doses for me. GT1>GT3>GT6?).
I can drive in 3rd person in GT and when i tried the demos of Forza i could not. I dont like driving inside the car (i do that IRL already thank you). I like seeing the beauty of the cars. GT is about Sexy cars. Thus i find it more appealing than Forza (even if since they have added a 3rd person camera. Dont really know).