I enjoy both american comics and japanese manga, and I frequently visit both /a/ and /co/.
Manga, well, you can say that it's all the same, but it's not. Of course, there are different genres, but a lot of the manga in those genres are different too. You won't say Princess Tutu is the same as Card Captor Sakura. Of course, there are still a lot of manga that are essentially the same, but you only focus on the good ones, right?
Same can be said about comic books. Yes, there are a lot of Batman, Superman, Spiderman, etc etc comics, but that doesn't say all of them are the same. Of course there are a lot of comics that have the same story in essence, but do you think comic book fans focus on those? No.
In manga, the books are produced by one creator, which is good, because you can expect an ending most of the time, and will be certain the art style won't suddenly change.
In comics, you get the story from many different views of different artists, and most of the time it's enjoyable to see. And because it's ruled by companies, any major decisions are always made by more people, so there's less chance it will be a bad one (not counting Spiderman's OMD).
What I'm trying to get at is: There are lots and lots of pros and cons in both manga and comics, so if we at least set aside the differences it's possible for us to enjoy both.