tough choice, if i really had to make a choice, i prefer manga over the anime because the manga is much more detailed than anime. and some animes storyline may be different from the manga.
One such good example is samurai deeper kyo. i like the manga very much. its very detailed and it covers alot of characters.
The anime was very dissapointing. it ended on 26 epsiodes. quite a number of characters in the manga never appeared in the anime.. and the storyline is quite different from the manga, actually its crap for those who had watch samurai deeper kyo anime and had read the manga. Luckily The manga is still on going :animesmil