One fact is clear- both Viber and Skype offer VoIP benefits.
The one question you are probably asking is, if you have an iOS and/or Android device and are on the lookout for using VoIP, in this case which to install. Unless you have intensions of installing both, you will be in a dilemma on what you want and what do you expect from your VoIP experience.
Popularity
Now if we speak of popularity it does not mean one app is better than the other in its category. However, when we speak of VoIP, the more popular app, more people are connected and talking for free, hence; saving more.
So in terms of popularity, Viber is definitely the runner up after all it has began way after Skype, hence; has lesser than a third number of Skype users. But, in time this ratio can change because you never know when tables turn.
Mobility
No doubt both are mobile applications. Viber is generally designed as a mobile app while Skype is designed as a desktop app and recently has progressed towards being an VoIP app with mobile abilities.
Data Consumption
VoIP follows on the logic of connecting people so they can communicate with friends and save money in doing so. If we speak of cost, yes desktop VoIP is much cheaper than a mobile VoIP, based on this fact mobile connection is costly.
Skype on the other hand also offers high-quality calls, hence; the data consumption of Skype is much higher than that if the Viber users.
Reach-ability
In most of the VoIP instances, the user can only connect to that person who is using the same VoIP app, meaning a Viber user can communicate with only another Viber user.
But, with Skype, the scenario is different, not only can a Skype user talk with another Skype user, but can also connect with a friend who has a Microsoft ID, whether or whether not on Skype. Plus if you require placing a call to another mobile or landline number across the globe, possibility is the cost will be low.
Ease of Use
The easiness of using depends on how the user intends to be. Now that Viber was intended to be a mobile app, it uses your cell phone number as your identification. However, recently the desktop version of Viber came and expectations were different, but the scenario is the same. So, the desktop using your cell phone number for identification is a bit odd to digest.
Skype from day one is a desktop app, so on launching the app you are required to enter your username and password. But, then again if you use it on the mobile and every time on launching the app you will be required to enter your username and password is considered cumbersome by most of the users.
Conclusion
For desktop users Skype is preferred and for Mobile users Viber is the better option as it will be easy to use.