It depends on what you want. C++ is its native language, so the won't be a middleman, which is a .NET wrapper you can use with C#, Emgu CV:
http://www.emgu.com/wiki/index.php/Main_Page[
^].
With C#, however, the development process is more smooth, goes faster, gives more reliable results as the change of accident bugs is much lower. Probably it would add some performance cost and extra dependency from .NET.
—SA