Define "better". If all your friends use one, then that is probably better for you. There is also functionality differences between the two (mentioned by others), which might force your hand.
That said, there are other quantifiable measures. In terms of security, Telegram has two main drawbacks:
1) it supports insecure chats - which means that for many (especially non-technical) users, they won't use the end-to-end encrypted messaging at all
2) whilst the encryption algorithms they use are standardised, the protocol they uses to transfer the messages is not well understood by the cryptographic community - and when it has been analysed, well-known flaws have been found [1].
In comparison, WhatsApp also makes use of standardised encryption algorithms, but also uses the Signal protocol - which has been studied by multiple groups, with better outcomes (such as [2]). One drawback to WhatsApp is they will generally make security decisions that are primarily based around avoiding sacrificing usability - if that is a problem, then perhaps Signal[3] or Wire[4] is a better choice.
On the encryption debate, using Russian algorithms vs US (Belgian) algorithms is somewhat academic - I believe that both are considered by the wider academic community to be strong when correctly used. I don't believe there is any evidence that Telegram or WhatsApp are incorrectly using them (beyond the attack found against Telegram, which may have been fixed by now?). This is the mostly the same for considerations about data storage location - if end-to-end encryption is enabled in Telegram, they are roughly equivalent.
That said, there are other quantifiable measures. In terms of security, Telegram has two main drawbacks:
1) it supports insecure chats - which means that for many (especially non-technical) users, they won't use the end-to-end encrypted messaging at all
2) whilst the encryption algorithms they use are standardised, the protocol they uses to transfer the messages is not well understood by the cryptographic community - and when it has been analysed, well-known flaws have been found [1].
In comparison, WhatsApp also makes use of standardised encryption algorithms, but also uses the Signal protocol - which has been studied by multiple groups, with better outcomes (such as [2]). One drawback to WhatsApp is they will generally make security decisions that are primarily based around avoiding sacrificing usability - if that is a problem, then perhaps Signal[3] or Wire[4] is a better choice.
On the encryption debate, using Russian algorithms vs US (Belgian) algorithms is somewhat academic - I believe that both are considered by the wider academic community to be strong when correctly used. I don't believe there is any evidence that Telegram or WhatsApp are incorrectly using them (beyond the attack found against Telegram, which may have been fixed by now?). This is the mostly the same for considerations about data storage location - if end-to-end encryption is enabled in Telegram, they are roughly equivalent.
[1] https://eprint.iacr.org/2015/1177.pdf
[2] https://eprint.iacr.org/2016/1013.pdf
[3] https://whispersystems.org/
[4] https://app.wire.com/