I always say prevention rather than cure. The newer cars (plastic) have the features to avoid crashes (prevention), such as ABS, traction control, ESP & BA (some models), etc., whereas the older cars (classic) don't. Also, I think the newer cars are much easier to handle, and respond better.
When it comes down to the crash though, sure, the older cars might have a tougher shell, but that isn't always a good thing, and doesn't mean you're more protected. The newer cars are designed to crumple away, and absorb the impact force (as Julie said) and the airbags and seatbelt pretensioners there to protect the occupants. The older cars don't absorb the force of the impact, and therefore pass it on to the occupants, who get thrown around more, and can have serious injuries (esp. 'cause there's no airbags)..