Is eltoo more memory efficient for watchtowers?
Without eltoo (ie today) a watchtower needs to store a different justice transaction for every channel update in case the channel counterparty broadcasts (deliberately or accidentally) an expired channel state.
With eltoo (once/if we have SIGHASH_ANYPREVOUT enabled onchain) a watchtower will only need to store a single justice transaction in case the channel counterparty broadcasts an expired channel state. The watchtower can delete old justice transactions.
This presentation from Christian Decker at Chaincode Labs is a great resource on eltoo.