They are clearly both abridgements of a larger work. Hermann Diels, in his great Doxographi Graeci (1879), discovered that the 5th-century CE theologian Theodoret had full versions of the quotes which were shortened in the abridgements. This means that Theodoret had managed to procure the original book which Pseudo-Plutarch and Stobaeus had shortened.
Both are summaries of a primary source. Hermann Diels, in his extensive Doxographi Graeci (1879), discovered that 5th-century CE theologian Theodoret had complete versions of the quotes which were shortened in the abridgments. This implies that Theodoret accessed the original book from where Pseudo-Plutarch and Stobaeus made their reductions.