I haven't used the Moodle 2 Comments Block, but I have been using ShoutBox with great in-class success. Just reading the Moodle 2 docs, it appears to me that Comments Block is much more static than ShoutBox which is really more like (and works much better than) Moodle's core synchronous Chat module.
In addition, ShoutBox lets users edit their contributions in real-time even after they have posted them. As the teacher, I can save and post these real-time chats. Very useful for synchronous working through concepts and examples.
Does the Coments Block have to obey the 30-minute edit rule as in a Moodle forum?
It seems to me that ShoutBox offers something much different that static asynchronous comments. I do hope I can continue using ShoutBox in Moodle 2. And I hope Moodle 2 can improve the core Chat module.