Which is better? quicker? more scenic? cheaper?
Driving is faster, but the train is better. There is a hefty fee for picking up a rental car in one country and dropping it off in another. And a car is a burden, not an asset, in the Cinque Terre.
We have done the train from Nice to Cinque Terre. Works great and was still quite scenic.
If I am not mistaken, the drive would mostly parallele the train ride, so there would be no reason to drive. The train is really a nice way to travel and let someone else handle the stress. Enjoy, Jeff