Well you've added some context in which they are comparable (some types of image processing), and I agree they are comparable given that specific context. I think that probably was your point that everyone else didn't get, but your original comment was void of context and implied some level of interchangeability, but you have to keep in mind that they are used for far more than image processing (even GLSL is), and will have very different results and very different implementations for the subset of tasks that can be accelerated by both.