+ with IRC I always lose the history, also, my client (Lingo on OS X) always disconnects and I can lose answers really easy too

+ I tried Slack in the past, and found no advantages or something that buys me

+ Gitter is quite buggy for now

However my personal preference is Gitter, because it give history, edit abilities and nice code appearance for free (though I wish to see light theme for code highlighter as GitHub provides by default).