I would use the native library for each OS i.e. libstdc++ on GNU/Linux and libc++ on Mac OS X.
libc++ is not 100% complete on GNU/Linux, and there’s no real advantage to using it when libstdc++ is more complete. Also, if you want to link to any other libraries written in C++ they will almost certainly have been built with libstdc++ so you’ll need to link with that too to use them.
More info here about the completeness of libc++ on various platforms.