Signal is a messenger for sequrity enthusiasts, Telegram is a privacy-respecting messenger for average user. Quality of Signal apps is not that good in comparison, Telegram is just much, much better for everyday use. It has native (not Electron) open-source clients for all popular platforms, tons of features and stickers (valuable for average people) and conveniently synchronises between devices. It does not have goal of monetization, but respects privacy of users and does not cooperate with governments. Has anti-blocking mechanisms built-in.
Based on past actions of Pavel Durov, founder of Telegram, there are no much reasons to not trust in integrity if his intentions so far.