Well others have looked long and hard at the available options out there, so you could take a look at what they found. The OpenSolaris and Java projects for example have moved or in the process of moving to Mercurial, but they did some research beforehand.
Requirements:
http://www.opensolaris.org/os/community/tools/scm/dscmreqdoc/
They evaluated Bazaar, Git and Mercurial:
http://opensolaris.org/os/community/tools/scm/bzr-eval/
http://opensolaris.org/os/community/tools/scm/git-report-final.txt
http://opensolaris.org/os/community/tools/scm/bzr-eval/