There are currently three ways of embedding data in the Bitcoin blockchain in use.
Inscriptions are written to an unexecuted branch of a taproot leaf script. This data appears in the witness stack of inputs. Witness data is only validated once when a node first validates a transaction’s scripts and is stored as part of the full blockchain record. When a pruning node performs IBD using the assumevalid option, witness data doesn’t have to be downloaded or evaluated at all. Witness data is discounted by 75% compared to other transaction data, but is malleable until a transaction is confirmed.
OP_RETURN data appears in the output script. OP_RETURN outputs are only validated once when the node first sees the transaction. The data in output scripts is not discounted and not malleable, as signatures on the inputs generally commit to the exact output scripts. As OP_RETURN outputs are unspendable, they are stored as part of the transaction in the blockchain data, but do not get added to the UTXO set.
Some schemes (e.g., Stamps) embed data in payment output scripts using either fake public keys or fake hashes. The data for output scripts is not discounted. As these output scripts are not clearly unspendable, these output scripts must be retained in the UTXO set.
Citrea recently announced that they plan to write data to payment outputs because they need non-malleable transaction data that gets confirmed timely in excess of the current OP_RETURN standardness limit. As the use of payment output scripts cannot be easily prevented, encouraging them to use OP_RETURN outputs instead would be harm reduction as at least the data would not be written to the UTXO set that every node has to retain forever.
The proponents in the mailing list thread and pull request further argue that it is more expensive to write large data payloads to OP_RETURN outputs than inscriptions and therefore use cases that already use inscriptions are not incentivized to use OP_RETURN. The break-even point for that appears to be 143 bytes above which payloads are more expensive to be embedded into OP_RETURN outputs.
assumevalidoption, witness data doesn’t have to be downloaded or evaluated at all. Witness data is discounted by 75% compared to other transaction data, but is malleable until a transaction is confirmed.