"The" is the determiner for the definite article.
"These" is the plural of the pronoun this.
In your two examples, you have two plural nouns - "books" and "twins", and as you show, either can be used. It has nothing to do with the comparative degree.
When you use the definite article "the", it has to be clear who/what you are referring to. This would normally be established in the context - for example, you might have named a particular set of twins in your preceding sentence, in which case it is clear who "the twins" are.
When you use "this" or the plural "these", the specific person or thing you are referring to is normally close at hand, being indicated, or currently being experienced. For example, if you took someone into an art gallery and asked "which of these paintings do you prefer", it would be clear you were referring to the paintings right in front of you, and you would not necessarily have to have named them verbally first.
So, either of your examples could be correct in different circumstances or contexts.