123FlashChat is a chat server that would run on your site, that people can log into with their existing social media accounts. The more people that are logged in, the more of your server's resources and bandwidth would be used.
The twitter option would be much lighter and wouldn't require running a chat server on your site.
Another option could be to embed a friendfeed post into your page. This would allow for real-time conversation right on your page for anyone with a friendfeed account. There wouldn't be a character limit on users for how long their comments could be, either. They could exceed well beyond Twitter's 140 limit. It's kind of like the best features of a chat room combined with the best features of a commenting system.
Here is an example of what it looks like when embedded into a page: http://kochlab.blogs...ding-speed-data.html
And it's pretty simple to do. You just make a short post on friendfeed and then click the share link on it. At the bottom of the popup box will be an embed code to paste into your page.
To make it look better than the example I linked to, don't include an image, just a simple text title for the post and link to the page it will be embedded on. The reason for including the link is because of the nature of friendfeed, where commenting tends to spread the content to a user's friends on the network, who may want to see what they are commenting on and join in on the conversation. (consider it free advertising)
I have also seen this used on a few sites for podcasts to allow live feedback from listeners at the time of the live broadcast, and then left up and open to be added to by those that download the archived version. The nice thing about it is that you get to keep the full history of the conversation embedded into your website and not just the last so many comments.
Twitter isn't very good if you want to keep all
the related tweets. They kind of restrict it to the most recent ones, and I think once the tweets are more than a week old, they disappear.