I'm really not into having the whole Which One Is Better debate, because I don't have a strong aversion to any genre: if it's a good book, it's a good book, yay books. YAY BOOKS.
Anyway! Despite the title, ultimately, the essay is more about the differences between the two genres, and more especially about the strengths of SF/F:
You absolutely cannot obscure underlying weakness with waffle. Otherwise the emails will arrive, picking up on discrepancies. Not just for the sake of point-scoring or nitpicking but because fans become so engaged with imaginary worlds and so passionate about their characters.
That passion, so easily mocked by laughing at Trekkies and Whovians, is another thing that distinguishes SF and fantasy from literary fiction. Mocking that passion is missing a key aspect of speculative fiction. By drawing readers in large numbers, contemporary fantasy becomes a platform to debate key, current social and political challenges, while science fiction continues to explore the impact of technological developments, for good and ill, before we have to tackle these things in reality.