Gran Turismo 4 is probably the best car simulation available on any platform. Ever.
If you're new, controls may take some getting used to but they are much easier than Forza Motorsport's "driving on ice" controls. If you're a veteran or have played past GT games than you'll be able to pick up and play this one quite easily. There is still Arcade mode but you'll hardly use it with the Gran Turismo mode which is basically a career mode.
As for new features, expect about 700 vehicles and lots and lots of tracks to choose from. There are tons of cars, past, present and concept cars. The tracks are real tracks from real life and every car is licensed.
The graphics are photo realistic and some of the best graphics ever on the Playstation 2. The cars look extremely well and the frame rate runs very smooth. There is tons of detail in the courses and overall it's just a spectacular looking game. The soundtrack is large with tons of songs. Some are from well known bands but others aren't. It's an okay soundtrack but it could've seen some improvement. The sound effects sound very well.
This is a huge game in terms of length. It will take you a long time to do everything. Try the endurance races they will take a long time. There is lots to do and tons of cars to buy and customize. It;s just a huge game and worth a buy. Think of all you're getting for $19.99
The new B-Spec mode is interesting. Instead of racing you are observing races. It is nice but A-Spec, the actual you racing parts are much more interesting. You can also take pictures and save them onto your memory card. The game now has online play too which is fun. I suggest you buy the "GT Force Pro" driving wheel from Logitech. It has force feedback and makes the races a lot more fun and seem like you're driving in real life. However, this wheel is not cheap. It's around $150-200 unless you find it for $10 at a Goodwill like my cousin and I did.
Overall, Gran Turismo 4 is worth all the money you'll pay for it. (Which is only twenty dollars.) To settle the arguement, Gran Turismo 4 is better than Forza Motorsport.