In SN5.8 the Bhikkhuni Sīsupacālā is asked by Mara which creed or philosophy - pāsaṇḍa in Pali - is she the adept of, to which she answers to instead approve the teachings or message - sāsana in Pali - of the Blessed One, the Buddha.
I would like to know if anyone has some more information on the etymological differences between these terms - pāsaṇḍa and sāsana - either in the context of Pali literature or in the context of Indian spirituality.
Is there anything special about the term sāsana that makes it something not to be blindly accepted or advocated for and instead examined and verified by oneself?
Is there anything specially negative about the term pāsaṇḍa that makes it something sectarianism, heresy or blind faith relates to as a creed or philosophy?
It’s a very rare term, congratulations on such an obscure question!
So far as I know it’s only used in the passage you cite, in Thig 7.2 (where it is addressed to a bhikkhuni Cālā, so is probably a retelling of the same events as in SN 5.8) and in the Vinaya at Bu Vb Pc 32.
The Sanskrit dictionaries appear to only attest the word in later sources (Mahābharata, Manu, Puranas) and have a purely negative sense, “heretic”. However, in the Vinaya passage a relative of King Bimbisara announces his intention to provide a meal for all the pāsaṇḍikas, so it cannot have such a negative sense. It must mean something more like “creed, belief, sect”.
Nevertheless, it is not a word applied by Buddhists to themselves, and each of the three cases it is used by a non-Buddhist.
As to the etymology, neither the Pali nor Sanskrit dictionaries offer any help. Perhaps it’s connected to saṇḍa, a dialectical term with the sense of “heap, multitude”.
As to why the Buddhists prefer to use sāsana, it’s hard to say. It may be because of some specific meaning or connotation of the word, but we lack any real evidence for this. Sometimes it’s just the accidents of history and usage.