I was used to managing all my packages in a Subversion repository, but now when we moved from Subversion to Mercurial in SymPy, I somehow prefer Mercurial for all my work now. :) We chose Mercurial, because I found it a little more polished than git, also the commands are the same as in Subversion.
I like the git-buildpackage, that it manages everything in one repository and that's it, and also that it's written in Python. One would expect that when Mercurial is written in Python, that hg-buildpackage would be too, but it's written in Haskell, so I cannot easily fix it. I filed a wishlist against the hg-buildpackage, whether it is a good idea to provide the same interface as git-buildpackage.
So for now I am going to try git for packaging things in Debian.