I agree, Never Let Me Go is an incredible book, so much better than the film. However, it's a very different book to Frankenstein. Never Let Me Go is a dystopian novel which you'll want to compare to others (for example, I wrote an essay with a comparison to A Clockwork Orange and even that was difficult).
If you're going to be writing about Frankenstein, choose another gothic novel. Or maybe even a novel or poem that contrasts images of science and religion in a similar (or contrasting) way to how Shelly does in Frankenstein.