After owning both I'd personally say Forza 4 is much much better. Better cars, lots more muscle cars for sure. And better engine sounds, plus engine swaps is one of my favorite features from Forza. Being able to take a Cobalt SS and swap in a 2000 LS1 is pretty damn sweet. Plus the exotic car line up is light years ahead of GT5, and the cars visuals are definitely a bit better. Sounds are not spot on, but still more realistic than GT5. The best for sounds I'd say was NFS Carbon I believe it was. That game had actual audio sound clips, and listening to the RX7 or the STi proved how real they sounded. Too bad NFS started changing things after that.