I think there are benefits to both! I have found that WhatsApp is extremely useful for practicing writing skills. There is also an audio feature of this app that is very functional and user-friendly to use on a mobile device.  On the contrary, Skype is extremely useful for students that want to practice talking face to face with their teacher. There is also a messaging feature in Skype, but I have found Skype to be much more difficult for some students to use. They both have benefits for the students and teachers!