I have read my fair share of both American comic books and manga, though, in all honesty, I prefer American comics. Unfortunately, it seems, I'm one of the few who does because everywhere I go, the majority seems to prefer manga. Before I start, I'm going to begin with the pros and cons of each, then I'll delve into the other aspects.
I have heard this statement from countless manga fans, that American comics are "just superheroes" and have little to no variety. Some of the more mainstream and well-known American comics focus entirely on superheroes, mainly Marvel, which many manga fans are unaware has inspired many Japanese artists. (Though, keep in mind, the artform is not confined to one country in general.) People automatically focus on the mainstream and then base assumptions solely from them alone. American comics are diverse and have different styles and plotlines (sometimes more-so than manga). The reason why I prefer American comics to manga is because it's generally more simple and easter to understand (even while having a unique plotline) and does not often overcomplicate the situation of the main characters, which manga tends to do. (I'm not saying that American comics are not as guilty as manga in doing so, however.) I'm probably a bit biased (though, a lot of manga fans are biased, too), I've grown up with newspaper comic strips (which has sadly declined in the latter years, due to the internet). Garfield, Peanuts, Calvin & Hobbes, The Far Side, Dilbert, Luann, Pearls Before Swine, and more have all been favorites of mine throughout my early childhood. I've also more recently started reading more graphic novels. Bone, for example, is funny, fast-paced, well-written, and just, well, epic. It's a Looney Tunes-esque sort of Lord of the Rings, which is why I like it because it manages to blend in cartoony, slapstick humor with drama and violence, making it all the more epic. I've also gaiend interest in Amulet by Kazu Kabuishi, another great story. I've also read The Adventures of Tintin (a classic), thus far, I've really enjoyed it. I also recommend Amelia Rules. I've read many more, but let's not overcomplicate things.