I have completed Sega GT 2002 and I must say I liked it more than I did GT3.
In SGT2002, there is just more freedom. In GT3, you don't have a "real" garage. In SGT2002, you have a "real" garage and it lets you buy furniture and other things for it. Your trophies are all around and pictures of your races are hung in frames on the walls. It's all pretty neat.
The racing itself suffers I guess you could say. The realism is not there like it is in GT3. I found that many cars pretty much all handled the same. The oversteer and understeer on cars was easy to detect but things like braking and cornering were all the same. It's not an obvious thing but you will notice if you're looking for flaws.
Races themselves give more variety. You can either go into a full "SGT2002 Mode" and play the whole game or just play some event races for quick money. (Drag racing is really fun.) If both of those are something you don't feel like doing, you can just play aracde mode or go into a classic mode.
Another cool thing about SGT2002 are the rewards. As you progress through the game you win abilities. From the start of the game, you can just go into the parts or used parts shop and buy a stage 3 turbo. You have to earn the ability to buy a stage that high. You will unlock it and then you have the ability to purchase that modification.
If I remember correctly, non of the above mentioned things were in GT3. I feel that SGT2002 is a better game simply because it has a greater level of depth.
That was my 2 cents.
