Jodee: I don't get where you're getting those numbers from, from what I've seen during the live it was about an equal number on both chats, but the IRC one was indeed the most active.
Also twitch chat is the one requiring a registration to participate to, IRC chat can be joined without any kind of registration. (we even provide a webclient link that allows you to join IRC without needing to download an IRC client).
The IRC chat allows us a better moderation in need (we haven't needed it so far, but in case, we would be quicker to react).
I must also admit it's a matter of habit, Yann and I are "seasonned" IRC users and it felt more natural to us.
There is one registration needed on the uReddit class page, it's a global way for us to see who is interested in the course. I agree it makes one more account which isn't very useful for students, but it's a nice way for us to get an estimate on the following of the class (and possibly could make it worth making other courses of the like in the future).