forza 3 is the only game on consoles that is arguably better than GT5. it doesn't offer the selected tracks night racing, two or three rally stages, licensed WRC, licensed NASCAR, one or two F1 cars, open-wheeled cars or selected tracks weather, but it does offer a more robust online experience, a livery editor, a functioning in-game economy that's held up suprisingly well, 480 cars with functioning cockpits and 20 cars with cockpits, but gauges that do not work, engine swaps, more robust upgrading options, all of the cars that raced in the ALMS GT for 2010 (except the new C6.r GT), all of the cars in the DTM, all of the cars in the V8 supercar series, all of the previous generation NASCAR stock cars except the toyotas, all of the factory LMS prototypes, most of the SUPER GT GT500 cars (only missing the new HSV-010 GT) and 32 porsches (not as important to some people, but that's what makes the game for me).
the only real competitors on the console are shift (which is horribly unrealistic and offers poor variety), GRiD (again, unrealistic, also now three years old) and F1 2010 (essentially features one car), and none of those are really competitive with FM3 or GT5, so to answer your question, just FM3, and it's not so much better as it is "greater than or equal to".