Does RCSI use less or same space than SNAPSHOT? Why?
Read committed snapshot isolation (statement-level snapshot isolation) potentially uses less storage than snapshot isolation (transaction-level snapshot isolation) because usually the row versions aren't required for as long a period of time after they've been generated.

For transaction-level snapshot isolation, the versions have to stick around for the entire duration of a transaction, which could be quite a long time (or quite a long time, as seen here). Of course, statements can run for a long time, too, but it's usually the case that a transaction will take longer.

To emphasise the point: A transaction running under snapshot isolation is guaranteed to see committed data as it existed at the start of the transaction. This means row versions have to be maintained until the whole transaction completes. Where the same transaction is running under RCSI, row versions can potentially be released after each statement. Overall, snapshot isolation tends to result in a larger version store size than RCSI on the same workload.

The same versions are generated with either or both settings enabled. I believe Kimberly was referring to storage impact in the sense of write rate, not storage used, so the two statements you mention aren't contradictory; just a little unclear or unspecific.