Straight gameplay wise, Forza is better in every way. That said, GT5 is much prettier, but its in no way shape or form 6 years worth of work. If it weren't for the Logitech wheel, it would be just another racing game. They also didn't put the same amount of work into every car in the game, and anywhere else except photo mode the cars just look "ok".
And no, I am not a Forza fanboy, I have had every GT except 4, and I feel like I wasted my money on 5.