I'll play both, but DEFINITELY playing solo on my first play-through. No doubt about it.
Even if I had a person equally dedicated to listening and reading EVERYTHING and think about each decision very carefully, I'd never have the same experience with another player in the party. I want the first experience solo, and then many more experiences both solo and co-op
EDIT: Besides, there is no individual among my friends that would be able to play as much as I want to. I can play for *MANY* hours straight without needing a break - the only one that can remotely match my stamina is my brother, but we both prefer to play solo at first and simply discuss the parts that we've both discovered as we play. Then, once we're both done with our first characters, we'll play co-op.
