I wish people who make those stupid comparisons at least try to understand how these games work. I'll try to explain. Both GT5 and Forza 4 use different car models for menus and actual races. In GT5, the highest detail models are used in the menus and in photo travel. A less detailed model is used when racing. And when you enter photomode during races, you still take photos of this less detailed model.
However, Forza does things differently. They also use the highest detail model in menus and less detailed model on track. BUT, when you enter photomode during races in Forza, it swaps the less detailed model with the high detailed one. By doing that, people who see photos of cars in Forza assume that the game uses the highest detail models during races, but that is not the case. (It would mean that the Xbox can run 500 000+ poly cars during racing and the PS3 can't.) So the video above is misleading. ( I hope someone corrects me if I said anything wrong here).
If you want to compare models between the two games, you should use the highest detailed ones from both of them and that means, in GT5's case, the photo travel ones. But still, in my opinion Forza has better tyre details ( not talking about physics, just visual representation) and seems to have slightly less blocky polygon edges. But cars from both games are already very close "copies" of actual cars so these small errors can be easily corrected.