Mill's major contribution to utilitarianism is his argument for the qualitative separation of pleasures. Bentham treats all forms of happiness as equal, whereas Mill argues that intellectual and moral pleasures (higher pleasures) are superior to more physical forms of pleasure (lower pleasures)
I would think the really big issue that these two sentences do not address is whether Mill was arguing that these qualitative differences are in any way "absolute" / "intrinsic" and apply to all people at all times or whether he was clear that the differences are subjective/ relative to the individual under particular circumstances.
That would be a huge distinction. I hope to read up and find the answer myself but if anyone knows where he clarifies this please feel free to post.