I like both, actually. =| They both have their perks. Normally, I'd go for the manga because I tend to like books more than movies, but I've seen plenty of anime I love. I can't really choose. They're both great in their own ways.
I've watched much more anime than I've read Manga. There's only one were I've read the Manga and watched the Anime, and because I get such a preference to the first way the story goes, I thought the Manga was much better. Well, there was also the fact that the Anime messed up someone's name and had a big inconsistency right at the beginning. I'm not sure if it ever got patched up, I only watched 1 episode, because it had terrible pacing, too. So, I guess the answer is, whatever I watch/read first.
I personally lean to anime. I don't need to hold a book and can relax while I watch it. Generally I watch dubs so I don't even have to pay really really close attention. I can do one or two other things while watching it and not miss anything.
Anime or manga. Both have their advantages. I prefer anime simply for the fact that they're shows. Moving animations with voices and actions. However, manga almost always last longer than their anime counter parts. Inu Yasha, for example, which I'm sure everyone here has heard of and seen at least one episode. They figured the anime was getting too long, maybe due to financial reasons or whatever, and cut it off. The manga continued long after, but I think that ended a few years ago. Bastard!! is another example.
Typically, when there's a manga, I'd rather read it than watch the anime first. Sometimes the anime is better, however, my favourite anime happen to be standalone as in there was no manga prior. That being said, it usually depends but I feel confident in saying that I prefer the manga. As an artist, I really respect the writers/illustrators work.
Also, manga is convenient in that I don't need to focus all of my attention on it, or I can take it with me very conveniently. Since it's a book, there leaves a little more to the imagination.