Google reader - you work 'within' your main browser. With FeedDemon, you add another browser just for reading the feed and open the links in feeds. So I have IE7 and Firefox and FeedDemon - open at the same time (first 2 I can't skip).
FeedDemon - one thing which was decisive was Offline Capabilities of the reader. With the Google Gears, though you add offline capabilities to Google Reader, most of the time the synchronisation in case of Gears, is a painfully lengthy process. And even after waiting for long, you are really not sure of its completing in the end.