Wow! Just 1 word. Wow! I'm sorry to say but after playing 3 hours of the Forza Demo, that's all I can say. So I'll give you the look I have on Forza Vs. Gran Turismo 3.
1st off. They don't even compare. Forza knocks it off in A.I., simulation, and details.
A.I.-These AI make GT3's look like dumbies. They are incredibly smart, and act like a real person learning from your every move. If you cut them off, they will just wheel around you. Coming back on to the track? Going around you. Unlike GT3s where you'll be smacked head, these racers move and go by. They take different lines every time, and change speeds every lap through the same corner.
Graphics-So it's not real like GT4. But GT3s are quite better. Forza's though seem to be plain racer friendly. People who love racing games, and don't care whether its sim or not will love Forza's. Quite a nice upgrade from PGR2, I went ahead and compared Forza's 360 Modena to PGR2's. PGR2's handle nothing like Forza's. This was how I could tell Forza was simulating racing a 360 Modena. They handled nowhere near the same, and of course, Forza's felt much more driving like.
Simulation
I would compare GT3s cars to one of Forza's but unfornately, the Nissan Z was a concept and was different to the Nissan 350Z. So there aren't any cars in GT3 I can compare to Forza's except one. The Toyota GT-One. And again. Forza's felt like racing the real thing. You see, to me, after driving a R390 and then a GT-One in GT3, they actually....felt like driving the same car, one with just more power.
Forza's felt way different. The customization part. Yes! You feel a difference. All the game gives you is an Exterior upgrade, and you will tell the difference in racing from just a Body kit, and Spoiler.
So this is how the overview goes:
Graphics-GT3 wins this one in realism, but Forza gets it in Plain Racer Friendly.
AI-Forza definately takes the win on this one. They won't smack into you.
Sim: Forza. Forza's felt alot different. And realer.
A difference in Braking!?
Yes. There is a big difference now. In GT3, you could hold the brake all the way down, and of course, you wouldn't see the wheels lock up. Not HERE buddy!
Forza's requires TAPPING people. Not, hold the brake down and watch the car slow down. You either hold brake down halfway or tap it. Braking is where I noticed the BIGGEST change from GT3. This is requires thinking on where you brake. You tap to fast, and you'll come into the corner too slow. Racers will take this to their advantage, and go around you.
Braking here is going to take a bit of time to master after playing GT3 to much.
Closing Statement:
Forza did what people did not expect it to do. It absolutely blows GT3 out of the water. Even the View that look like GT3's Front View has 2 views. One like GT3's and one resembling inside a car. You can tell the difference, as the car doesn't sound as loud, and other cars are louder. Engine sounds are matched perfectly, and this game reminds me so much of GTR's demo.
This game has truely stolen the racing part in my heart. It IS a Must BUY or Die (j/p) title for the Racing Lover! As far as I'm concerned, there doesn't look to be anything that needs improving on from just the demo.
All it needs now is....an Official Forza Wheel.
Seriously, I don't see why there's such hot debates (or any debates at all) going on between Forza and GT4. I mean, let's think realisticlly here. Who HONESTLY thinks that a game just taking it's first few baby steps (in this case, Forza) can topple the fourth installment in a proven, solid line of electronic racing pedigree (in this case, GT4)? I for one sure don't. But that's NOT to say that Forza will suck. Having played the demo myself, I'm quite satisfied with what's been done, and even if I didn't play the demo, I'm pretty sure that my confidence in it would still remain roughly the same. Though whether or not it'll be the Xbox's best racer is debatable (just because it's a simulation doesn't mean it outright beats the arcade-style offerings. It has to EARN that right, and I don't think it'll topple Project Gotham Racing 2 in terms of fun value when all's said and done), but there's no denying that it'll be "better than average". No matter the results of Forza, though, one thing is certain, and it's that we are, whether you like it or not, ARE looking at a long-term competitor to the Gran Turismo series. A competitor that'll dethrone GT or remain in it's huge shadow is yet unknown, but Forza IS in it for the long run, and for racing fans looking for an alternative, that's the beuty of the whole thing. You don't have to like it. You don't even have to appreciate it. But you'll have to live with it. 
