As far as I'm concerned, Signal is superior in every way. Telegram simply has gained a lot of publicity and increased its user base (which is critical for IM) thanks to E. Snowden.
A friend and I downloaded Signal to try it out, and just couldn't get anything to work. I'm on iOS 8, the app would just crash anytime I hit any button to make an action (send a message, even delete my account). On his end (he was on iOS 9), it said it connected to me, but his messages just got lost in the ether in between us.
I mean, I'm a technical guy. I don't understand what either of us was doing wrong. Have you seen anything like this in your experience?
Only if literally the only thing you're looking for is security.
A big deal breaker for me is being able to use it without a phone number, and from desktop/sim-free devices, as well as from multiple devices simultaniously
People seem to use it as a normal IM client. They have a web UI and don't really claim to be all that secure (eg not trying to compete with Signal). I don't understand the benefit over the numerous other IM apps, everyone who uses it just says "oh i like Telegram better" or "my friends are on it".