Signal will always be the better, more secure chat protocol. The ingredients are just too different. I see Delta Chat as complimentary, reusing the mail infra and encryption tools we already had, packaged user friendly, skipping some harder concepts (web of trust, key signing) of pgp.
The encryption is sound, but mail itself leaks meta data at submission time, so a mail provider can make a difference, but it’s up to the user if anonymity is relevant (not only by email address). The message content is very much secure. If any of the messengers servers are down, Mail usually works and smtp will also do redelivery if the destination is offline.
good mail providers use transport encryption where they can, at submission and between mailservers and not record the submission IP. But apart from this, your provider doesn’t matter much. You encrypt on top of the transport with the public key of your recipient and vice versa
if you purge your mail account: the options to delete messages from the server but keep them in the app after a set time exist, and/or delete them from the app too. I think the common use-cases are covered by now
That being said, the centralized services can offer more features like webrtc audio/video calls that DC would need infra outside of mail for, you’re able to record asynchronous audio messages though.
Delta Chat increasingly tries to market itself as “mail chat”, a good idea. If both ends use the DC App too or another Autocrypt compatible client, they will encrypt “e2e”. But using unencrypted plaintext mail in a chat view has also its charm, a good client can make a difference. Convince a friend and give it a try!