A ramhorn will flow better than a cast and will ultimately be able to make more power, since cast manifolds are typically very short, log style and the runners are not equal length. However, there's a better chance you can keep ps and a/c with the cast, such as a Greddy.

If this is your first turbo setup, which it sounds like it is, just go with a Greddy kit. They're great beginner kits.