Personally, this ever-lasting debate between FM and GT will never reach a conclusion. Both games have very strong fan base.
I have played Gran Turismo series from GT2 on playstation, and played Forza Motorsport 1 on original Xbox. I loved both games equally, and I still love both games equally. GT has strength in its history, because it was the first racing sim on console that got popular. It contained tremendous amount of cars that made many car fans excited, and its simulation-like physics and car customization made the game a legend.
But Forza Motorsport was a great racing sim also. No doubt, it was inspired by Gran Turismo series. At the same time, FM tried to distinguish itself from GT franchise by introducing customizable layers for the cars, strong paint editing mechanism, and be able to add bodykits to the cars that actually has some effect to the cars instead of just being cosmetic. And damage modelling to every cars.
In that sense, I find both games very appealing, and that is why I played both games and at some point, and I will end up getting both GT6 and FM5.
Many people debate on graphics of each game, and I really think graphics is a subjective matter. One person finds lighting and shading effect of GT superior to FM while other person might think otherwise.
So I don't get into debate when it comes to graphics. It is simply pointless.
Personally, I do not have much faith in GT6 because it is coming out on PS3 as well. I am waiting for GT7 made specifically for PS4. But I will buy GT6 because it will be the best racing game on PS3