In my experience, Skype offers more functionality and greater reliability. That said, you could always book the italki classroom while sharing your Skype ID with your teacher via message. That way, if Skype gives you any trouble, you have the the classroom as a backup. In general, though, I’d be more concerned about the classroom not working, or not working well (there are often weird problems getting the audio to function on both sides). When it comes to Skype, problems are most likely on the student’s end, since we teachers use it all the time. For this reason, my students on the Chinese mainland often use the classroom. Teachers on the Chinese mainland, on the other hand, usually have their Skype working fine. My Chinese teachers have never had any issues, but ask first if you’re concerned.