All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
There is much good in each one of these two books. Each book has its own distinctive characteristics.
Al-Bukhari in Al-Adab Al-Mufrad, details each matter in a separate Chapter and he puts different subheadings in it, as is the case with dutifulness to parents and undutifulness to them; he detailed the chapter into many subheadings. Meanwhile, An-Nawawi may abridge the chapters to a certain extent. In Riyaadh As-Saaliheen, he starts the chapter with some relevant Quranic verses and then cites many Ahaadeeth from the different books of Sunan, so, it may be more comprehensive in this regard.
Moreover, Al-Adab Al-Mufrad, as its name indicates, contains Prophetic texts about etiquettes in general. As regards Riyaadh As-Saaliheen, it covered more subjects like good morals and dispraised conduct; it also contains a chapter on prohibited matters and so forth. So, it is more comprehensive in this regard as well. Shaykh Al-Albaani revised these two books and they are both translated into English, but the translated versions are not revised (i.e. they include the original Arabic texts before revision).
Allaah Knows best.