Mill claims that those who are suited to make a comparison between the value of two things are only those who are acquainted with both.

So, for example, I am not competent to judge whether the music of the Beatles is more enjoyable than the music of N-sync, unless I have heard both.

And on some matters -- call them matters of judgment -- this appeal to the judgment of competent judges is all there is. (Cf IP3 584a)