They would be diagrammed the same -- only the second one would have a mistake of case -- subjective i.e. nominative is required by the conjunction, not objective. "Than" is a conjunction -- not a preposition. This is at least true to prescriptive, as opposed to descriptive, grammarians.
1. He is better than I am. 2. He is better than I.
In the second sentence, is it a conjunction with a following ellipted clause after 'better', or, owing to to the redesigned syntactical environment, should I conceive of 'than' in 'than I' as being a converted preposition from a conjunction: 'than' followed by 'I' vaguely resembles a prepositional phrase? Vaguely only, as the form of 'I' is out of place. There is only one explanation for the grammaticality of #2 that comes to mind and I can accept: Do not rely on your eyes. Look behind the scenes.
Now look at this:
3. He is far from being happy. -- prep. (from) + noun (gerund clause) --> Alles in ordnung (Everything is fine). 4. He is far from happy. -- prep. (from) + adj. (happy)
Traveling on my train of thought further, the question comes up at the inspection of the fourth sentence: Is this correct to assert in a world where only those things exist which my eyes can perceive that an adjective can't follow a preposition? I am torn between thinking that the fourth sentence is incorrect grammar and thinking that 'happy', together with the invisible but understood gerund head (being), is there with sufficient justification.