In contrast to Signal, Telegram doesn't end-to-end-encrypt messages by default (they get stored in plaintext on their servers), it also doesn't protect the social graph and even stores your contact list on their servers. Even WhatsApp is more secure than Telegram.
They are more or less the same thing if you are not trusting anyone. Both require you have to verify the key fingerprint for a particular contact (safety numbers) if you want effective end to end encryption. Both are silos. Telegram is better about distribution and can be gotten from places you might trust better (e.g. F-droid, Debian). Both have some sort of reproducible build thing going on. Both could get access to your connections to other users if they wanted where Signal also insists on access to everyone's phone number. Telegram works on desktop without also insisting you have the program running on your phone.
