Bzr is now a GNU package, though it has not been updated to reflect
that.  It is most unlikely that git or Mercurial will be GNU packages.
Thus, unless Bzr is unsuitable, we should use Bzr.

We've seen enough to know that each has its advantages but all are
workable.  So we should use Bzr.  And if there are aspects of it which
prove inconvenient, we can ask the Maintainer of Bzr to improve them.