They're the same medium, but the execution of each form is drastically different depending on the influences affecting the art style and composition. Personally I've always found most manga to be very visually uninteresting, not to mention that as a gaijin I hardly ever know what to make of all the strange goings-on and find it hard to relate to characters and situations that have been so heavily mangled by translation and cultural distance. The American comic is something that has so much more depth because it's had all this time to establish itself and let its concepts and structure and imagery distill into something readily identifiable but still easily recognized, and typically I find that the substance of Western graphic art appeals to me much more.