It's neither better nor worse.
C# comes with a HUGE standard runtime library, and there is an enormous amount of high quality components available.
C/C++ gives you better control and more freedom - including the ability to easily shoot yourself in the foot without intending to. There are many high quality open source image processing and computer vision libraries available. Choosing the right one depends on your needs.
For a customer with the need for a high quality solution I would choose C/C++ for the core functionality. There is nothing that prevents you from using both C# and C/C++ in the same solution, so maybe part of the GUI could be done in c# and .Net.
If I needed something real fast, as in should be done yesterday, I would shop around for a .Net component suite - maybe
Leadtools[
^]
Regards
Espen Harlinn