For joint collaboration I've used subversion. Having a central repository I think is a really good idea for a paper, although you can set up central repositories in bzr and git.
I would stay away from git for two reasons, however. First, Windows support doesn't seem to me that great. You have to either use something like Cygwin or the port, which is hardly optimal. Unfortunately many people use Windows. The second is that it's a bit harder to use git. I love git myself but subversion is straightforward: there's commit (and conflict), whereas with git you have a local repository you can commit to and then you can also push to a remote repository.
In the end I like subversion for collaborative efforts. The only downside is if you really like to branch your work, then subversion is horrible compared to git or bzr. Bzr and svn also have nice version numbers ;)
