Not to be facetious but, one is C the other is C++ and on occaisons it does matter a lot.
OpenMP is good for Fortran style code, where you decorate loops, although I havent looked at the latest OpenMP standards. But on its own C++11 looks very good. I will (and I do) use it where I have uses of C++ template mechanism. But now I am porting some that stuff to D. Generics is so much less verbose and nicer over all.
Though I love the support for type inference in C++11, it does not play well with expression templates, which is my go to reason to turn to C++ in the first place (perhaps the only one prior to C++11).
I know there is a big difference between modern C++11 and C.
C11 has standard threads similarly with what you find in C++11, unfortunately you won't find this implemented by all vendors. I hope to see complete support for C11 threads in GCC and Clang in the near future.
Any performance reason for which one would prefer Cilk vs OpenMP ?