Let's quickly look at two concepts new to Feed Digest that RSS Digest doesn't have.
Searches. You can now specify a "search" on a digest. The search works somewhat like Google and means your digest will only contain articles from your specified feed(s) which match your search string. So if your digest reads from 50 different feeds, and you only want to show articles containing the word "google" but NOT the word "microsoft", then you'd specify a search query of "google -microsoft", and bam.. your digest only shows those items!
Global digests. Generally you'll specify a feed, or a number of feeds, that you want your digest to be based on. But what if you want a Technorati/Feedster-ish view of every feed in the Feed Digest system? With a global digest you can run a search over all new posts from the last 6 hours and use that as a digest! In my current testing I have a global digest set up with a search string of "london bombing", and it shows me every item from all of the current feeds which are about that topic. Check out a screenshot of the digest. Remember that that's entirely templatable.. you can make all that info appear in whatever order and using whatever HTML or CSS you like!