Bhagavad Gita · Raja Vidya Raja Guhya YogaChapter 9, Shloka 2 (9.2)

Six qualities: (1) king of sciences / king of secrets, (2) supreme purifier, (3) directly experienced, (4) in keeping with dharma, (5) easy and joyful to practise, (6) imperishable.

Shankaracharya. rāja-vidyā vidyānāṃ rājā dīpty-atiśayavattvāt — the king of sciences because of its surpassing light; pavitram … aneka-janma-sahasra-saṃcitam api dharmādharmādi samūlaṃ karma kṣaṇa-mātrād eva bhasmīkaroti — it burns to ashes in a single moment the karma stored up over thousands of births; pratyakṣāvagamaṃ pratyakṣeṇa sukhāder iva avagamo yasya — directly experienced, like happiness; dharmyaṃ — something with many virtues might still go against dharma, but this does not; susukhaṃ kartuṃ yathā ratna-viveka-vijñānam — easily accomplished, like the skill of judging gems; and while what is easily accomplished usually brings a small fruit, avyayam … na asya phalataḥ karmavat vyayaḥ — its fruit does not diminish as the fruit of action does.

Acharya Ramanuja — another meaning of "king". Rājñāṃ vidyā iti vā rāja-vidyā, rājāno hi vistīrṇāgādha-manasaḥ — "the science of kings" — kings meaning those of vast and deep minds — the science of the great-souled; pratyakṣāvagamaṃ … bhakti-rūpeṇa upāsanena upāsyamānaḥ ahaṃ tadānīm eva upāsituḥ pratyakṣatām upāgato bhavāmi — worshipped through devotion, I become directly present to the worshipper at that very moment; susukhaṃ … atyartha-priyatvena upādeyam — easy, because it is so deeply loved; and most beautiful of all: avyayam … mat-pradāne kṛte api na kiñcit kṛtaṃ mayā asya iti me pratibhāti — "imperishable" — even after giving myself to one who worships like this, I feel that I have done nothing for him.

The difference: "Direct" — Shankara: knowledge of the self, experienced as one experiences happiness; Ramanuja: the Lord becomes manifest to the devotee in that very moment. "Imperishable" — Shankara: the fruit does not diminish; Ramanuja: the Lord's debt of love is never repaid.