Comment on Comments WordPress SEO Plugin by SEO Dave.
SEO wise there’s not a great deal of difference between the Stallion SEO Recent Comments widget and the XX Stallion Recent Comments 12 widget. They are both SEO’d, unlike other WordPress recent comment widgets that add a link to the comment authors website with a nofollow link that deletes link benefit!
The new version (Stallion SEO Recent Comments widget) has more options, the gravatar and description, but the link part are identical, a text link to the comment only (no author link, no wasted link benefit).
The gravatar is loaded as a background image using CSS which SEO wise is neutral (invisible to Google), so having a gravtar/not having a gravatar with those two widgets is identical SEO wise. The description can be argued both good and bad SEO wise, depends on the site really, I use it for usability, users seem to like the comment links with description more than just the link, presumably as it gives an indication of what the comment is about so morelikely to be clicked on.