I don't see how these two apps can be compared. Teleg.eu is a feature-rich social media platform that you can use to stay free from the Facebook-WhatsApp monopoly.
Signal represents one feature of Teleg.eu, which is Secret Chats. If you think you need a separate app for that feature only, installing it might make sense for you.
Personally though, I find Secret Chats much more usable - and secure. After PRISM I have trouble trusting anything US-based, let alone cryptographers funded by the US government. I explained my reasoning back in 2017: https://teleg.eu/s/durov/59