I can notice the slight difference in physics between forza 4 and GT5 car handling, but I have no clue which is more realistic. I've never driven at those speeds in real life and probably never will.
Both are responsive and handle consistently, that's enough for me.
Night driving and weather is not space intensive, it is computationally expensive. The frame rate dips in GT5 when you are driving behind a bunch of cars in the rain with all the water spraying up. Dynamic lighting and the shadows that come with it also take a lot more out of the gpu then pre-baked shadows and lighting, hence the rather crude shadows in GT5.
Turn 10 did not want to compromise the graphics at all at any point, thus left both out.
GT5 has one big advantage to Forza 4 though, it has 1080 lines of resolution, 1.5 times as many as Forza's 720. This makes a big difference for me. It is much easier to see the road and judge distances with the increased resolution. There is not a lot of competition at 1080p, I only know of ridge racer 7 and wipeout HD that also run in full HD. (Technically GT5 is not full HD, 1280x1080 but the vertical resolution is what counts to see the road in racing games)
Bring on next gen, I want to play Forza at 1080p.









. Mario Kart is so funny... so heavly sales
... but that's the exception because no other racing game can do GT numbers.