What do you think about the latest WhatsApp update? Is now WhatsApp as secure as Telegram? Which one should be trusted? Which one has the strongest and most secure encryption?
This sounds like a job for your eyes! Read the terms and conditions and see if they mention anything about your data in there. Remember YOU are using THEIR service, which means YOU have NO CONTROL over what you put on their service.
Realistically though it sounds like it's in their best interest to keep your private stuff private since it's easier on them, easier on you, and safer for both parties.
However without checking the ToS of both apps in depth you can't really know for sure. This at best is no more secure than encrypted email services, and at worse is not secure but that is the nature of using someone else's encryption service and not your own.