If there's something that would drive me away from Mercurial and closer towards Git, that would be Git-TFS. So far I'm using a Mercurial/TFS solution, as explained in Using Mercurial with TFS. However, this is not a 100% foolproof solution because (argh!) the solution file contains the "scc*" settings that every now and then get removed from the file when it is not bound to the TFS source control. And, of course, this happens all the time when the code I'm working on is in a cloned Hg repository.
The light seem to come from a project called Git-TFS. The documentation is available at the Wiki page in the project. There is also a newsgroup where questions can be answered.
I'm about to give it a test run but, reading more about it, I seem to have an idea that not using two Hg repositories but just one might work, too. Update to follow soon.
Good introductory text is here. My initial opinion is quite good.