I am a Forza buff. I think it is absolutely amazing. Its a considerable upgrade from Forza 3. The Graphics are better. The spoilers work. The cockpits look more real. The physics engine is way more realistic. The AI is much tougher. you pretty much never leave a race without damage.
All I can say about GT5 after a long long lonnnnnnng wait, it didn't came out most of us hoped for. It was never about the graphics, it is more about physics. Don't get me wrong, the support from PD is unbelievable, but what really killed the project is the limitation of the hardware. Kaz has said it over and over again, he didn't want to release the game for another 2 years, but Sony made him. Previous GTs were great, but the overall view of the simulation was limited. Being GT5 is the only PS3 GT so far, it would be nice to see if they can follow it up with the same hardware. However, GT6 will more likely be on PS4 so that means going from scratch again.
I have not played Forza that much either so I really can't comment on this.
Asking fans of Forza and GT which is better is liking asking fan boys of Xbox and PS3 the same question.
Horizon has nothing to do with either the Forza and GT Simulation.