This section of the SN contains four chapters, which are about nāga, supaṇṇa, gandhabba deva & valāhaka deva. These four classes of beings (sattanikāye) are described differently in the following ways:
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Per SN 31.2 & SN 32.2, gandhabba deva & valāhaka deva are deva (gods) because they have performed “good conduct” (“sucaritaṁ”).
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Per SN 29.3 & SN 30.3, the nāga & supaṇṇa are so because their past conduct was “mixed” or “both kinds” (“dvayakārino”).
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Per SN 29.3 & SN 30.3, both the nāga & supaṇṇa are described as long-lived, beautiful and very happy.
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SN 29.3 says the nāga keep the sabbath (uposatha) so they can go to/be reborn in a heavenly world.
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SN 30.2 says supaṇṇa “carry off” or “plunder” (" harati") the nāga. It seems the supaṇṇa are more powerful than the nāga; yet both are powerful because supaṇṇa born from an egg can only carry off nāga born from an egg but not those nāga born from a womb, from moisture or spontaneously (etc).
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SN 30.3 does not seem to mention the supaṇṇa keeping the sabbath (uposatha).
We should keep in mind, for the most part, the suttas (e.g. AN 6.39) equate the godly & human beings with good kamma & equate the animal, ghost & hell beings with bad kamma.
Therefore, it seems a “nāga” is a powerful being yet between the animal realm & the godly realm because they perform mixed kamma. Also, nāga have the aspiration to go to a better place, such as heaven. Where as, possibly due to their power, the supaṇṇa are content to remain as supaṇṇa. 