Is Frankenstein a cautionary tale about the excesses of rationalism and science or a parable about the egocentrism of Romanticism and its fantasy of the artist as a god-like creator?

You could make a good argument for either of these proposed options; arguably, Frankenstein could be both of these things. It depends upon your interpretation of the text.
As one of the earliest Gothic novels, Frankenstein was born out of the Romantic tradition; its author, Mary Shelley, was the wife of famed Romantic poet Percy Shelley and a friend of Lord Byron. It has been suggested by some critics that Victor Frankenstein is, in part, based upon Percy Shelley and that his journey is a criticism of her husband's Romantic ideology and reverence for the individual self. In Frankenstein, the creation upon whom the artist has lavished all of his attention eventually dooms both of them; he is not a triumph but a selfish celebration of ego, and his creator never recognizes his own part in their mutual downfall. It is also notable, on a metatextual level, that the final accepted text of Frankenstein was itself heavily edited—some would say butchered—by Percy Shelley, exerting a final level of egocentric authorship over a creation not his own.
The above, however, is more of a fringe theory. It is more widely accepted that Frankenstein, written during the Enlightenment period, is a cautionary tale about the dangers of man playing God. Gothic turns a mirror inward to show society its own fears: within this text, the fear it highlights is society's concern that the development of science would eventually lead to the death of God and organized religion. In extrapolating from contemporaneous discoveries surrounding electricity and anatomy, Shelley depicts an extreme and monstrous world in which man comes to know too much and, like Prometheus, steals "fire" from the gods and dooms himself.