Here is the relevant quote about Anne's opinion: in Chapter 11 Anne discusses literature with Captain Benwick
" ...and feeling in herself the right of seniority of mind...
Frederick's thoughts and opinion can be seen in Chapter 20:
"I confess that I do think there is a disparity, too great a disparity, and in a point no less essential than mind. I regard Louisa Musgrove as a very amiable, sweet-tempered girl, and not deficient in understanding, but Benwick is something more. He is a clever man, a reading man; and I confess that I do consider his attaching himself to her with some surprise. Had it been the effect of gratitude, had he learnt to love her, because he believed her to be preferring him, it would have been another thing. But I have no reason to suppose it so. It seems, on the contrary, to have been a perfectly spontaneous, untaught feeling on his side, and this surprises me. A man like him, in his situation! With a heart pierced, wounded, almost broken! Fanny Harville was a very superior creature,.."