Hey Deborah! Which technique works best for your sculpted piece largely depends on its form. If it is a fairly simple form with a wide opening at the bottom you may easily get away with a block mold, but if it is more intricate and needs venting to release possible trapped air, then a 2-part-mold (box, matrix or brush-on mold) would be better.
If you have a photo of the piece to be molded, please feel free to post it on the forums so I can give you better feedback: http://community.cgcookie.com.
~Lisa
