Over at newsarama the debate continues on is manga more popular then American comics or is American comics winning the fight?
Anyway, someone pointed out the sales numbers of comics being around 400-500 million in the states to mangas 175-200 million. However let's keep in mind, in a given month, about a quarter of the comics released by Diamond are actually manga. So making half of what American comics make with much less material released means the demand is incredibly high.
As for accessibility and consistency, manga definitely takes the edge. It has a higher readership of women and young readers. I mean, most the time I'm at a comic shop I see guys around my age or older. At a bookstore I see all kinds of people reading manga.
Is manga more popular then American comics? I'd say yes. Look at it my way, it may not be bigger then American comics in the States, but from international POV, it kills American comics.