Hi
JT and Mary,
The differences as I see them.
Skype can be used to phone "contacts" be it Smart phones or Laptops etc. for a small price you can then phone landlines and standard mobiles. Some will support video calling if they have a front camera.
Viber uses "Numbers" when I installed it, Viber went through my contacts and found 3 of my friends already using it.
Skype needs you to enter a contact ie. BertBloggs234 (excuse me Bert Bloggs)
Viber will run in the background with no extra battery loss.
Not sure about Skype
Mary, in answer to your question, my wife has put Viber on her phone.............it will work WiFi to WiFi, Data to WiFi or WiFi to Data.
It looks like use what ever suits your needs.
Steve