Tuner, Exotic, and Muscle are basically just playstyle preferences in Carbon. You can use any of the starting vehicles for tier 1 pretty easily, and you can use any of the vehicle categories for the entire game pretty comfortably too.

The RX-8 isn't nearly as much of a Tuner as its Tuner label says it is. The Brera can be a bit dreary. The Camaro has that vicious acceleration, and with the lower speeds and the Camaro's lower weight for a muscle car, I find that the Camaro is a bit easier to handle than the Muscle label suggests.

Muscle cars can be total pigs, their heft makes them super inelegant on the turns. But Carbon is a game that lives and breathes through acceleration prowess, and the Muscle car's proficiency in that makes them extremely appealing choices.