Your point is well taken, but it may still be necessary to know others - for example, I work at a large and well-known Internet company and we use Perforce =)
Well, Git or Mercurial are equally powerful and both are DVCS (Distributed) which SVN is not. If anything, mercurial is said to be easier to learn if you already know SVN, but both are similar. Git use UNIX philosophy of many complementing tools whereas Mercurial uses a monolithic approach, a single unified powerful tool.
Go with Git. Although I never used Mercurial personally, I considered it at one point because it was in Python but someone else mentioned the fact that there is a version difference because of the split between Python 2 and Python 3. That can complicate things.