Telegram bills itself as secure, which comes with a rather large burden of proof if anyone is to take you seriously.
As the saying goes: "don't roll your own crypto, unless you're a professional cryptographer, and even then…". Telegram are not cryptographers but decided to roll their own anyway and have come in for not a little flack as a result.
Search for "telegram app crypto criticism" and you'll find plenty of examples of this.
Is Telegram secure? I'm not qualified to answer that, but I understand enough to be sceptical.
Is Whatsapp secure? If not, and they're both insecure, then I think Telegram makes a better choice, if for nothing else than its better multi-platform support.
I liked Telegram but all my friends were on WhatsApp, so stuck with WhatsApp for that. I agree on your point that Telegram does not send nasty letters like this.
No matter how bigger WhatsApp gets, I am glad there are some alternatives.