When comparing two things, you would use 'the better (not just 'better' but with the definite article).' For instance:
 This auto is the better of the two.
And when it's obvious you are only referring to two autos, you would just say
 This auto is the better.
Now, which is more idiomatic, "This auto is the better" or "This auto is better" (when there are only two cars in front of you)?