I got all three, and the Nissan..
There's not much in it really, I would say that between the Nissan, Bentley and Mazda it's purely a matter of taste - which one do you like the most..
Citreon is not in the same class, it's a different type of race car..
Most of the Le Mans style cars are pretty similiar, they all have traction issues in lower gears, they can all do 250mph, they all handle well compared to other racecars, but they are much lighter.
Personally, I like the Bentley and the Toyota GT1, but in terms of performance it's very close, there isn't really one car that is seconds per lap quicker than all the others on every track.
Each car has got it's stengths and weaknesses..
Like I say, it's a question of which one you personally like the look of.
In previous Gran Turismo's some of these cars were putting out 1200bhp, but that's not the case in GT5, it's been evened out alot more than before.