I don't even know what Telegram does, but if I have Signal and iMessage I'm not sure what anything can offer me that I'm missing...
You'll ultimately have a make a descision based on who you trust more.
Not a telegram fan at all but their clients are also open source
But Telegram has one big advantage - they publish a public tdlib library with which you can easily build your own Telegram client.
Very large groups, if that’s your thing.