Not saying that it's necessarily a bad option, but my biggest issue with delta chat is that it does not offer forward secrecy (if a user's private key is compromised, past messages can be revealed); Signal does. Delta no question beats signal in decentralization, though email is less decentralized than it seems--how many people do you know who still use gmail? Delta also inherently leaks metadata on whom you're communicating with to the email host (that's just imap/smtp). Signal can mitigate this somewhat with Sealed Sender (which gives one-way anonymity), though it can be broken with statistical analysis, and signal metadata is more identifying due to requiring a phone number.