WhatsApp doesn't let me login from two mobile devices. Can't use it from phone and tablet at the same. It's moronic. Telegram does, so...
Whatsapp messages pass through their servers, but do not remain there. They are effectively sent from the sending device to the receiving device. This means that your messages reside on your device. When you use the desktop or web client, it's basically remote operating your phone.
This famously allowed Whatsapp to achieve massive scale with relatively few resources. [1]
Telegram messages reside on their servers. This allows for features such as those discussed above - multiple synchronized clients etc. The downside (aside from potential privacy concerns) is that it requires much larger amount of infrastructure, which presumably is one of the reasons they need $700 million.
[1] http://highscalability.com/blog/2014/3/31/how-whatsapp-grew-...
And now they've had a decade to address the issue.
Telegram is not E2E encrypted, WhatsApp is.
Telegram has the option of E2E encrypted messages, but that then works only from one phone/PC.
I have never used backup for WhatsApp but just the other day, out of the blue, every conversation from late 2017 to now that I had deleted popped back into my inbox.
Can’t explain that one easily