You should get them both is the best answer I can give but if it has to be one or the other choose the FGT.
FGT
+ Harder to master so more fun. Practical due to the fact that you can use it in A+B spec and will find more online action with it. Comes in a variety of colours. Cheap to buy.
- Car doesnt exist in real life.
Ferrari
+ Authentic car. Relatively easy to drive. A must for F1 fans.
- Very expensive to buy. Limited to Ferrari F1 races only online. Can only be used in 1 player mode on 1 seasonal.
Bear in mind that you can win either Ferrari or FGT for completing B spec Endurance races, I spent 10 million on a F2010 then promptly won one. Now I have 2 and cant sell one either