Been reading the last couple weeks about rookie Dmen here. Orpik's name seems to be a big topic of discussion. With rookies, it's not fair to any of them to look at the numbers for valuation purposes (the bad numbers AND the good numbers). It's more about how they handle themselves at this level, and how they have improved over the course of the year. So I would say that Orpik has done well in his rookie season, given the type of defenseman that he is. Made rookie mistakes and still has a lot to learn, but he did a good job according to his role.
However, I still don't think that he was the best defensive defensemen amongst the rookies. I still have to say that Exelby did a better job. Remember, both played on bad defensive teams, though Pittburgh was a bit worse. Exelby is just as frightening in the open ice as Orpik. Not the most important aspect for a Dman, but it does make the other players a bit wary after a while. Exelby is better at reading and reacting right now, making the smart play. Orpik has had more problems with that this year, of course that could and probably will change in the next year or two. But simply in terms of this season, Exelby has been the more reliable of the two while the physicallity has been pretty much the same.
I don't have the first clue if one will outshine the other in the coming years. That won't matter too much to Pittsburgh and Atlanta, respectively. They are both very happy to have these two young talents.