GIT, because it has the best performance and are backed by large projects like the Linux kernel, Wine, etc.

May I add that Mercurial has Python dependencies, while git are written in C.

If I were to bet who outlasts who, I would put my money on GIT. Also, considering GIT is so simple by design, I foresee it will be used in many other cases than source code management. As it is just a matter of creating new high level porcelain, like say, revision control. ;) How about that Linus?