Based on what little I've seen, I'd say Castro.
I'm just happy that the cubs are finally getting some position player prospects that aren't complete and total busts. there have been so many guys who were supposed to be great, Patterson, Choi, Pie, Gary Scott, Brooks Kieschnick, the list goes on.
Now the Cubs have had a few position player prospects come up and make an impact, Theriot, Soto, and now Colvin - Castro.