I'm searching for compelling Git and Mercurial clients on Mac OS X. The most clients I've found so far were less compelling as I expected. Some of the clients are programmed even in Ruby or Tcl/Tk, which IMO aren't good OS X citizens in regard of integration in the OS.
I have clients similar to Versions.app or Cornetstone in mind, which are Subversion-only clients. Perhaps somebody got an insider tip for me.
I just thought I'd mention that SourceTree is a Mac OS X client for both Mercurial and Git, in one tool. I wasn't sure if you were looking for that, or just mentioned both because you hadn't decided which to use yet; personally as an open source developer / user, having both available in one tool is very useful to me (that's why I wrote it :))