The Sandilya and Narada bhakthi sutras like the Bhagavata are fundamental works of mysticism . Sandilya bhakthi sutras seem to be older on account of its archaic tone and is evidently modeled on the pattern of the great philosophical Sutras. Narada Bhakthi sutra quotes sandilya but the sandilya does not quote Narada.
Narad Bhakthi sutra surpasses not merely Sandilya by its easy eloquence and fervid devotion but it may be even regarded as one of the best specimens of Bhakthi literature that have ever been written. The Sandilya-sutra is more philosophical than Narada-Sutra. It goes into the question of nature of Brahman and Jiva , their inter-relation, the question of creation , and so on.
Both the Sandilya and the Narada quote the Bhagavad Gita on the one hand, and later Bhakthi literature on the other.