Beyond the fact that security on Telegram is a joke (E2EE not enabled by default, only available in 1-to-1 chats, groups chats are all unencrypted, homespun encryption algo), they have never had a full, independent audit of their encryption standard.

It looks like there are a handful of papers that looked at parts of the earlier standard Telegram used (MTProto 1), but nothing on the current version (MTProto 2).

https://courses.csail.mit.edu/6.857/2017/project/19.pdf

https://eprint.iacr.org/2015/1177.pdf

https://eprint.iacr.org/2015/1177.pdf

Anyway, long story short, Delta Chat has had independent audits several times. I'd say that says it all, really.

https://delta.chat/en/help#security-audits

(Also, thanks for introducing me to Delta Chat, was unaware of the project up to now. Neat stuff.)

I've never seen anyone use Telegram's e2ee. Not even by the users outside the legal realm, to put it mildly. Not only is it opt-in but it also works in the mobile app only.

Telegram is not private. That makes the comparison to be infinity in favor of DeltaChat.

Regarding privacy, PGP is far better than out-of-the-shelf IM-embedded encryption, if used correctly. Alice uses Bob's public key to send him a message, and he uses his private key to read it. He uses Alice's public key to send her a message, and she uses her private key to read it. No one can eavesdrop, neither governments, nor corporations, nor crackers, no one except for Alice and Bob. I don't get why someone would complain about "usability", for me, it's perfectly usable. Commercially available "E2EEs" (even Telegram's) aren't trustworthy, as the company can easily embed a third-party public key (owned by themselves) so they can read the supposedly "end-to-end encrypted" messages, like a "master key" for anyone's mailboxes, just like PGP itself has the possibility to encipher the message to multiple recipients (e.g. if Alice needs to send a message to both Bob and Charlie, she uses both Bob's and Charlie's public keys; Bob can use his own private key (he won't need Charlie's private key) to read, while Charlie can use his own private key to do the same).

If you have to choose go for PGP. However, there are much better options