What's the difference between Viññāṇa (consciousness) and Sañña (perceptions)?

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consciousness is raw sensory data, perception is higher on the food chain. For example, MN 18 and DN 22.

SN 22.79 explains it with a confusing simile

cdpatton notes:

The Chinese parallel (SA 46) doesn’t have these definitions [from SN 22.79] for perception or consciousness. They look more standard.

Perception

is defined like this: 諸想是想受陰，何所想？少想、多想、無量想、都無所有作無所有想，是故名想受陰。 Perceptions are the clinging aggregate of perception. What is perceived by it? The perception of few, the perception of many, and the perception of measureless. When there’s nothing at all, there’s the perception of nothingness. Therefore, it’s called the clinging aggregate of perception.

For consciousness,

the definition is to be conscious of all the sensory objects: 別知相是識受陰，何所識？識色，識聲、香、味、觸、法，是故名識受陰。 The signs of discernment are the clinging aggregate of consciousness. What is it conscious of? It’s conscious of form and conscious of sound, odor, flavor, touch, and mental objects. Therefore, it’s called the clinging aggregate of consciousness.

Dr. William Chu comments on same SA 46 passage

諸想是想受陰，何所想？少想、多想、無量想、都無所有作無所有想，是故名想受陰. 

"All the perceptions is what the clung-to aggregate of perception about. What are these perceptions? The perception of singularity, the perception of multiplicity, the perception of infinity, and the perception of 'there's no do-er whatsoever and there's not a thing whatsoever.' That is what is meant by the clung-to aggregate of perception." 

別知相是識受陰，何所識？識色，識聲、香、味、觸、法，

"The clung-to aggregate of consciousness is about [the ability to] differentiate-discern characteristics (note: the literal translation of vijnana is to differentiate-know). What is differentiated and discerned? Sights, sounds, smells, tastes, tactile feelings, and mental contents [are differentiated and discerned]. That is what is meant by the clung-to aggregate of consciousness."

Now, as for the question about how perception is different from "differentiation as in by consciousness." For one thing, the "differentiating" in vijnana/consciousness is, in ancient Indian Upanishadic usage, about the bifurcation into and reification of subject-object duality. To be a conscious being is to experience a "oneself" that stands in opposition to an environment (world). In Buddhist usage, an added meaning is often found: the experience of continuity (therefore consciousness is intimately connected with the retention of memory and karmic continuity). Here's a way to summarize: Being conscious is being aware of senses and their corresponding objects; being conscious is also about experiencing continuity--to have memories and to be able to anticipate the future.

To be a percipient being, on the other hand, is to experience "specific contents" such as "there is a multiplicity/diversity of experience...there's only One-ness...there's not-a-thing." In Buddhist cosmology, all sentient beings are conscious, but not all beings are necessarily percipient. An example would be a human in deep sleep, an asanna being in the non-percipient state--both are said to be conscious without experiencing specific contents. A meditator in the nirodha state only experiences the temporary abeyance of feeling and perception, but is still "conscious," and therefore retains the continuum/memory of the person when he comes out of the samadhi. Quite a few Buddhist treatises speak of beings with 5, 4, 3, 2, and 1 aggregates, always with consciousness being the indispensable aggregate.