Depend on what you are shooting, the motorcars or the babes.
If you have the space to move around to compose, prime lens will give you the bokeh
and sharpness that you desire.
However, for motorshows usually they are crowded with photographers especially when you are shooting a popular and beautiful model. The zoom lens will give you more chances of getting your shot. The 18-55 and 55-200 VR are good lens to start with.
Try out a 50 f1.8 first, and see if that suits your needs.