I have both games because I have a stupid love for cars and could afford it. That said, if I had to pick a favorite it would be GT5. I love Gran Turismo because it lets me experience the cars. Drive them. Enjoy them. In Forza I don't get that attached feeling because no matter how nice the game looks, or how fun it is to paint the car, it does not feel the way it should. My Karmann Ghia in GT5 drives EXACTLY as it does in real life. I play GT5 with the Logitech wheel, surround sound, a 100inch 3D HD projector and it is like nothing else.
That said, everyone is different. I purchased Forza so I could race with cars I don't have access to in GT. Mainly the random super cars assortment. I do enjoy the paint features, and the visual customization option, but for me that's the end of the excitement with Forza. I tried the Kinect features and did not like that at all. Admittedly, since Forza doesn't support my wheel I opted to buy the wireless wheel that MS makes. Its a better experience than just using the controller, but its nothing compared to a real wheel. When you are racing it just doesnt feel real. It feels fun, no doubt, but not real. I enjoy both games but for anyone who wants to experience driving, I would point them towards GT5.