i'm not sure, depends what i want and how i want it.
i prefer the manga because you can understand whats happening in a fightscene. Sure, in an anime they can make sure you understand, by slowing it down so much that it just seems like a choreographed dramatic ballet scene or something... which would be fine if i were wanting to watch something that wasn't action-blood-rushing-awesome paced... but most the time i don't. In manga, the fighting isn't moving, but in your head, you can visualise it all happening at a preferred speed, with all the techniques being visible etc.
Another point agaisnt Anime, again a fighting transition problem, would be the talking. yeah sure, in the manga, a monologue explaining the technique is acceptable, because they could be talking as fast as they are fighting... okay, no they couldn't, but shut up, i don't tend to think about it! in anime though, both characters stand still, or in that "swordagainstsword shaking staredown", awaiting the opponent to finish talking... which just breaks the flow slightly.
However, if i'm in an emotional mood, i would watch anime any day over reading manga. sure, stories and backgrounds touch me in mangas, but i have never cried reading a manga, no matter how emotional i was feeling. Anime though, it can make me cry... i guess i connect more with the characters or something, there voices being not contained in a speech bubble. so yeah, it all boils down to what mood i am in.
Regardless though, i will do both. I will watch all those anime episodes i watch the day an english sub update is released, i will read each manga chapter on the day of release too. i can't have one without the other, don't make me choose.
EDIT: wasn't going to mention favourite animes/mangas... but its so hard not to. I used to love Bleach most, and i've watched every single episode ever released. and by doing so, i've noticed a decline in quality, which has upset me. I think the animators have gotten lazier/reduced in number, and now the fights aren't nearly as impressive to me anymore. i still watch it though. You would have never heard me say this before, but i much prefer watching Naruto or One piece or FMA to Bleach... they all seem to have taken a step forward, whilst Bleach took one back. perhaps just my opinion, and there are problems with all of them (such as One Piece having far too many scenes where it switches one by one between 40 different characters with their mouths open, making gagging noises due to awe/fear of seeing an attack ¬_¬)
I don't know what people mean by FMA, do you mean this new one thats coming out? Full Metal Alchemist Brotherhood follows the story of the manga, and i've been following it for ages now and it is still awesome (though the english dubs are sounding pretty lame).
Naruto has really impressive fight scenes... probably the best current day anime i'm watching at the moment... though i wouldn't say its my favourite. One piece snatches that easily XD
I also read Hunter x hunter (which started again recently
yay), History's strongest disciple kenichi, Claymore and many others, which are all brilliant
