Just to correct one minor misconception -- I have yet to use e-mail with darcs. (Other people have sent me a patch or two, but I've never used the send feature myself.) I always use ssh/scp. E-mail integration with darcs is a feature, but not one that's necessary to use.
I haven't had a chance to use bzr-ng seriously yet, although one Python OSS project I'm involved with is switching to it soon-ish. My current belief -- which is probably obvious ;) -- is that darcs is quite nice for small projects, while svn is best for large projects. bzr-ng presumably is fine for both, but I think it has some sharp corners still.