Security: WhatsApp is the famous app when it comes to user privacy and security. Telegram claims to encrypt every messages sent through it. The algorithm used for encrypting is so strong that even top scientists can’t crack it. Telegram has a secret chat feature with which, you can set timer for messages. The chat history and messages will be deleted forever from the phone as well as Telegram servers after user specified seconds.
Ratings and users: WhatsApp has 20+ million ratings and it is used by more than a billion people whereas telegram has managed to get rated by 13,00,000 out of its 50 million users
Pricing: You can us telegram free for life. Unlike WhatsApp, there’s no subscription charges for this application. WhatsApp users should pay just 1.2 dollars after 1 year of usage to continue using the app.
Advertisement: Both apps are free from banner, floating and other types of ads. According to their parent company i.e Facebook and VKontakte, WhatsApp and Telegram will remain ad free.
Stickers: Telegram offers a few more stickers than its competitors. Its users can upload their own smileys using the stickers bot module.
Attachments: Although there are a lot of online services that can be used to send files, sometimes using instant messengers is more convenient. For example, you can use Google drive or dropboox to share large size files with your friends Unfortunately, you are allowed to share just pictures, videos, location in WhatsApp. With telegram, you can easily share all types of files without worrying about their size.
API access: You can use the telegram api in your app. WhatsApp API is not public yet. It allows third party developers to use just sharing function with which, users can share links in groups, chat messages.
Conclusion: Both apps are good but as WhatsApp is used by more people, you should use it. Unfortunately, Telegram is popular only in some countries. Thus you might not find your friends in this app. I was able to find just 2 telegram users from my 200 contacts.