Nothing I can add as to which to use. But I could explain the situation at ZW.
Zelda Wiki.org, uses Skype as its chat, both for general users and staff. We found that it was simpler use and more dynamic. Our previous attempts at IRCs failed because no one wanted to use them. This was actually the idea of myself and Mases. We saw some other sites doing it, so we started one up. It's the main chat for it now and we've found it far more effective than an IRC, our official IRC channel was closed down shortly after starting on Skype.