After Hours of playing here is my conclusion
PS: Yes I have also finished Forza( I own an x360 too).
Anyways, my very first impression of the game was highly disappointing, I saw it as a HD remake of GT4, but wait I rated it 9.0, WTF? Here is how this game grew on me, and this is how it was meant to be understood.
GRAPHICS:
At first your will mostly play around with only "Standard" cars, which will fell let down, since the standard cars basically look like a HD GT4 rep-off.
Good things about Standard cars:
-The car reflects ANYTHING, even the other cars and the road, its simply well designed.
-The lighting, and design of headlamps for any car be it Standard or Premium, are just damn real.
-In rally all cars get changed as the race progressed (ie dust, snow etc.), viz a standard for rally games, but not a race sim. (I dont remember if this was there it GT4)
Premium cars are like GOLD, they are just too damn good. Insane number of polygons and damn well made to be insulted with.
Premium cars:
-Bloody insane detail
-The detail is only limited by the on-screen resolution, even on HD.
-Damage physics are perfect.
World details:
-Some tracks are basically BAD(eg, carting tracks are just plain pathetic)
-The famous tracks are, well designed (despite the copy/paste jobs noticeable)
-Geometrically very accurate, and to me thats all that matters.
SOUND:
Perfect. The changed in pitch when tuning the engine and the turbo and supercharger sounds are simply perfect! I can't point anything out, except for the fact that hitting a wall still remains with the same THUD sound.
GAMEPLAY:
Major improvements have been made. Also with the PS3 controller your can control the throttle well, with the L2 and R2 buttons. You can even control the high&low vims on the head-lights, and also the windscreen wipers.
Major changes:
-There is an active clutch system, which basically means that your car doesn't just give 100% torque, rather it give max torque which can be generated at the given situation (ie in a power slide, or drifting)
-Drafting is a major part of the gameplay now, as drafting behind a car improves your acceleration and hence allows you to overtake, rather than just rely on your cars's HP.
-The new controller, helps to keep the FR over 500BHP vehicles from drifting when exiting a corner.
-Tuning is similar to forza, but no vinyls.
Photomode:
Nothing big about this, if you have tried the same in GT4, just the fact that now its HD, and the premium cars look sublime.
B-Spec:
Disappointing, really. I thought you could do so much, but your just ending up with 4 commands, which even a 5 year old can master.
THE KILLER:
The only factor that i can say really destroys the beauty of this game are the shadows. The are just littered with bugs and artifacts, which destroys the whole atmosphere. If the was fixed, the game would surely make a different review.
Online:
OK, not something brilliant (even though it hasn't really had much of an update since release). Laggy clients can really spoil a game.
Compared to forza (the big question):
For me, froza was a real let-down, despite all the gorgeous surrounding, your not going to be staring at a butterly when racing at 200KMPH?, but also it was to linear, no variation at all, not even an endurance race, or rally, it was simply. I'm not calling that a racing sim, but for what it worth, the little fun i had with it was very good, but after 10-20 hours in, i dropped the controller and just could take it.
GT5, has that variation, carting, NASCAR, rally, B-spec(even though its dumb). drifting & endurance races, and to me thats the pulp of the experience of a racing sim.
To understand GT5, dont judge it in 2-3hourse, play for 3-4 days, you'll know what i mean, when compared to frorza.