I read both.
Right now I'm following Ultimate Comics Spider-Man, the follow up to Ultimate Spider-Man after the Ultimatum Mini Series, and Ultimate Comics X, a comic following the story of Wolverine's Son after
during the Ultimatum mini-series.
click to show spoiler
Woverine's Death killing Magneto for Killing millions of people world wide with a tidal wave
Before that, I read the entirety of both Ultimate Spider-Man (133 issues), And Ultimate X-Men (100 Issues).
But I also read a lot of manga.
I own the hard copies of the Complete Azumanga Daioh (4 volumes), Ai Yori Aoshi (17 Volumes), all current Neon Genesis Evangelion (11 volumes), I'm missing the last volume of Full Metal Panic (8 volumes out of 9). And after I get back to work, I have every intention of buying the rest of Chobits (I have 3 out of 8 volumes), Love Hina (2 out of 14 volumes), Tsubasa: Reservoir Chronicle (4 out of 23 volumes), and several others I've been collecting for years.
There is no reason why one cannot be a fan of one and not the other. To say that one is better then the other is shear idiocy, as ones opinion on the matter is just that, someone's opinion which only matters to that person alone.
Comics and Manga come from two different cultures, and honestly, that doesn't matter, since they are exactly the same thing: Pictures pieced together to form a cohesive story bound together in a printed book.
And comics here in north america aren't all super heroes with long capes and super powers, let me introduce you to a man named Frank Miller, whose works like Sin City and 300 are anything but. Going back to the super-heroes, let me introduce you to Alan Moore and his cult classic Watchmen which defied and spat in the face of what was seen as the traditional Super Hero.
The argument about which is better is moot, since both appeal different groups of people and cultures, and there is a alot of them who love both, myself included among them.
To me, there is no argument. Both are valid forms of art. Both stand equal.
But that's my opinion.