Hmmm. I love both.
Anime. I'll say, yes, you could use your imagination to picture what you want them to sound like, but if you heard the voice of the character, sometimes.... you won't get disappointed. And it's nice to see animated characters moving around and interacting within the story. Sometimes, the anime's story does changes, for better or worst. Sometimes, the quality of artwork could sometimes improve, but nothing's perfect.
Manga. You get a more indepth look into the story and characters. Your imagination is your weapon here, since Manga gives you a lot of freedom on what you can think of (such as your own background music, character voices, and so on.) The art quality is usually good in manga.
My final view is that... both are good, they're like Salt and Pepper. But since I'm here anyway... I'll vote.... manga. I sometimes like salty stuff. =P