So I own both and can give a verdict of my own.
Graphics: Inconclusive, Am running GT5 on an HDTV and FM4 on an old CRT TV. Forza looks good though, even in 480p.
Sound: Forza, no doubt. V10s have the distinctive exhaust note that is not seen in GT. You can hear the howl of a V12 and the high RPM buzz of a rotary. In GT5, the cars sound like blenders (but the transmission and turbos sound better in GT than FM).
Car Selection: Tough decision, GT5 has more cars (although I heard FM4 had 1000 liscensed cars, its more like 500-600) and yes, I know that GT has the Veyron as a (gasp) standard car, and that there are 45 different types of Mazda Miata, but GT has more cars and a better spread. Also, GT is still coming out with new DLC every month (two months?) and new cars coming with each update. GT5 wins this challenge.
Tracks: GT5, more tracks but less layouts. Also has the underwhelming course maker, at least it makes new tracks, even if they are not quite what you wanted.
Driving Dynamics: (settings: all aids off except for ABS) Tie, Forza has better over/understeer on cars, but gets way too slidey when you floor it in any car (didn't think you could get such a big slide in a Nissan Leaf did you?).
Forza has the vibration in the controller, which is good for telling you the wheels are spinning, but some cars just spin the tires endlessly (Maxed out Spark FWD, 568 hp) and race tires do no good in controlling them, and your hand gets numb. ABS is impossible to work with in FM4 and very easy in GT5. GT5 also has over/understeer and spinning tires, but much less, plus, race tires actually give good grip. GT5 has much more realistic smoke from the tires (thick and hard to see through). GT5 is more controllable and much easier when driving at full speed or when drifting (very hard in Forza). However, Forza can be more fun, driving a 1000 hp camaro at 250mph is just more exciting when you know the wheels could slide out from under you at any moment. Forza is exciting (huge slides) , GT5 is rewarding (fast lap times)
Tuning: Forza, GT5 has good tuning but FM4 takes it farther. GT5 does not have things such as drivetrain or engine swaps, there is only 1 type of supercharger and 3 turbos (low, mid, and high RPM).
Ease of Use: On a small TV, GT5 is infinitely better, large buttons and text, matched to menus that use the whole screen are much better than Tiny bars on the side of the screen (usually cut 1/2 off on mine. anyone know how to change the useable screen size?) and miniscule text that is impossible to read.
Overall: GT5, more cars, more tracks, a more rewarding and easier drive, and easier use prevails over good sound, slightly better tuning, and all premium cars.
Nice try.
👎
) and also they might.... emphasize might include night racing...oh but wait its only available on a few tracks. Actually its not really night its 7pm because we at Turn 10 fear that if it was any later like 7:30pm then the frame rate will drop below 59.9FPS and make the game unplayable. Its also why we did not include weather, dirt, more smoke, more than 12 cars, decent AI and any other notable features 
