It's a tricky situation really, as some tracks are more technical, thus mixing it up a bit. You also need to be in a club with the four cars unlocked for them to be available, and they are all good, believe you me.
Like torque says, the Golf will get you everywhere in the hot-hatch races. Above that though is where it can depend on the track. The Bentley club car is good with long straights, something worth remembering, but less good on the tighter tracks.
You could always put some time into single event TT or race and find out which suits your style best, plus that helps level you and your club up to get more cars.
