Eyal,
As soon as you use one single line of C# it makes no sense anymore (beside of existing code or very unsafe code) to use C++.
It is up to you if you use C++, Java, C# or VB mostly is that all personal preference from the one who uses it.
In the case of C languages like C++ and Java and C# it is even slight because those are all mathematical (Algol) based program languages.
However, for me is important that I get with .Net an seem to be endless repository of classes which I simply can use.
Success
Cor