Anne finds confidence, ardour, enthusiasm, and wit as alluring -- but she is also attracted to literary-minded and musical people. Fred combines all these qualities -- he is also well-read and musical. Lady R is well-read but not musical. Capt Benwick is well-read (we don't know if he's musical or not), and with gentle manners, Anne gets along well with him, and defends his manners to Admiral Croft "to oppose the too-common idea of spirit and gentleness being incompatible with each other" (chap 18). It seems to me someone like Capt Benwick would appeal to both Anne and Lady R -- "a very, active, zealous officer", well-read, with gentle manners.