I agree with the general sentiment that the red fort and agra Fort are virtually indistinguishable. No doubt helped by the fact that both have significant historical overlap in their structures.

Weirdly enough, I remember agra Fort as more interesting than the red fort, but when checking back the photos I tend to like the red fort better. Overall I think both have a place in the list as examplary complexes. Imo the most diferenciating aspect, and one that I particularly enjoyed was the persistence of some akbarite architecture in Agra. In this sense I think Agra Fort showcases the changes in earlier mughal architecture much better than Delhi.