From what I can recall, the new "S" version has a different (more powerful) processor, and also will allow you to run the newer RoomOS rather than the old BoardOS giving you a better upgrade path / "future proofing".
All newer boards being made are the "S" version, so, the reason the non-S may be cheaper for you is it is likely to be old stock.
12 May 2019 is the last ship date for the old hardware version - see Cisco announces a change in product part numbers for the Cisco Spark/Webex Board 70
Wayne
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