These kinds of questions ultimately end up being subjective. It’s a bit like asking, who is a better painter Camille Pissarro or Paul Gauguin? Your personal taste factors in, even with realistic racing simulators.
Eurogamer‘s Digital Foundry took these comparison images, so props to them for that.
In the gallery, the images marked “PS3” are Gran Turismo 5 screens. The images marked “Xbox 360” are Forza 4.