For many people, the answer depends on what apps the majority of their contacts use. Both apps have free audio and video calling to other contacts, groups, broadcast messages, and can sync with multiple platforms.
But that is where the similarities end.
Viber has more features than WhatsApp. I mentioned the Secret Chat, Viber Out (Viber-to-Phone), and getting a local phone number. Another nice feature is called My Notes, which can be used to set reminders, a to-do list, or just something you don't want to forget. The note is encrypted, just like a message, and backs up across all connected devices.