The widget displays image links (icon buttons) to various subscription services and social networking sites. Upto 40 services supported.
There are two differences.
The 'Feedburner Feed' option displays the Feedburner icon while the 'RSS Feed' option displays the standard RSS icon (as do 'RSS Feed for Posts' and 'RSS Feed for Comments' options).
For 'Feedburner Feed' you only need to provide the Feedburner feed name in the widget options and the URL for the link will be automatically generated, whereas for 'RSS Feed' you need to provide the full URL.
The general 'RSS Feed' option (or 'RSS Feed for Posts' / 'RSS Feed for Comments', as applicable) can be used for a Feedburner feed when the standard RSS icon is preferred over the Feedburner icon.