For me, it's heavily subjective on the particular series.
If we're talking about a very action-based series like Naruto or Bleach, the anime definitely helps you visualize what's going on in an otherwise hard-to-follow fight scene.
I don't read or watch very many action-y manga or anime, though. I prefer the comedy/slice-of-life/romance/4-koma genres. And in my experience, the manga for those types tend to be more enjoyable because of their humorous asides and comments outside of the speech bubbles. But because those comments hardly ever carry any meaning outside of that one panel, they're often neglected in the anime version.
However, those types of 12 episode comedy animes usually result in character singles and image albums, which is a HUGE selling point for me because I'm such a big fan of those types of songs.
So all-in-all, unless the anime can provide a few more layers of enjoyment that is absent in the manga (the animation style, the voice work, the accompanying music), I think I'd have to say I prefer manga over anime. But just barely.