On my system FeedDemon uses 97% CPU and 45 MB RAM when it updates the feeds. GreatNews uses 10% CPU and 20 MB RAM. However GN finishes the update faster than FD. I did the test with the same 130 feeds. But as it has been said FeedDemon is that hungry on CPU cycles only when it updates the feeds. But FD has the upper hand in some instances, for example the cleanup wizard is faster and more stable than on GN.
FeedDemon going free means also that it will be updated and not abandoned, in a world where Google Reader is actually an RSS Reader Killer. Newzie hasn't been updated in almost a year and the pace of development on GreatNews has been reduced considerably in the last year (due to work + kids, says the author of the software on his forum). But all the 4 RSS readers mentioned by me are mature products, there are not many essential features that could be added and the major bugs have already been fixed.
The only feature that would be really useful is the addition of an Ad-blocker, to use with the built-in browser based on IE that is included in all the RSS readers.