Signal for mobile and Signal for desktop are different apps with different code bases. Neither is as good as Telegram's, in my opinion.
Signal is fine for messaging. Not bad, not amazing. I'd have a much easier time convincing people to switch to Signal if it would've had a client as good as Telegram's, especially for the desktop application.
That said, Telegram has been adding more and more annoying premium features that distract and annoy.
On iOS, if you turn off your Internet connection and receive a message, you won’t get a notification when you restore your connection. This problem doesn’t exist with WhatsApp, Messenger, Instagram. Quite strange.