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Plasma withholds the data. A rollup is supposed to publish it.
Both use the parent chain as a court. They split on whether a user can reconstruct the state when the operator is hostile.
Why the exit matters
Plasma commits to a child chain and lets users exit through a fraud proof. If the data was never published, the user may not be able to make the proof. A rollup posts the data, or a proof about it, so the state can be checked and left. Cost and who may upgrade the contracts are the next questions, not a slogan about scaling.