What's the difference?
See http://www.darcs.net/DarcsWiki/DifferencesFromSubversion , http://www.tummy.com/journals/entries/jafo_20060102_172958
I think the relevant differences (with resp. to ManaMana) are:
Less space requirements in some situations
Darcs is able to create a repository that doesn't include a physical copy of every file, which may be useful for keeping your repository on a usb key, for example. (see darcs get --no-pristine-tree)
More powerful merging
Darcs is based on a theory of patch algebra, which allows merging to be more flexible in a lot of cases. i.e. re-naming variables
Branching in darcs is simpler
A new branch is just another local copy of the repository. You can selectively move patches between branches, and to/from the server.
Every workspace is a repository
There's no distinction between a workspace and a repository under Darcs. So InterWiki under ManaMana needn't employ a central root wiki. Continuations could be squirted around promiscuously a la UseNet, but without expiration. Or hived off to special purpose wikis with their own concepts of OnTopic. All while keeping the central wiki concepts in global sync through merging. WikiDom unite!