They're both pretty much the same speed since they're both compiled to CIL
. A lot of the features of the languages have also been unified, so the difference is just in the syntax.
However, I think knowledge of C# is more benefitial since a lot of the concepts are applicable in C/C++ as well. It's much easier to develop in C#, but if you need all the speed you can get, go with C/C++.