Well, why not using both 😉 ??
I am using (literally in C# … lol) EMGUCV.NET (which is an OpenCV C# wrapper) and AFORGE.NET at the same time:
AFORGE.NET plus its “Image Processing Lab” makes much sense for filtering options (edge detection, thresholds, and so forth) and easing viewing functionalities.
OpenCV provides all the rest like SIFT/SURF and other more sophisticated image processing routines.
That’s why you use .NET: simply bring all the components you need together in one application 🙂