I prefer to keep the two separate. My linkding is used to dump pages I may revisit someday or not where readeck is stuff I want to definitely revisit.

As far as features go, the big one for me is that Readeck can export to epub. I don't use it a ton, but it's nice to be able to load an article on my ereader.

I use linkding for bookmarks and readeck for read-it-later