They really bring different things to the table, so in the ideal world... both. Play whomever depending on the matchup.
However, given that the Cs are to be without Perk for probably half the season, it's important to get someone who has a game similar to his. I think Shaq fits that better than Brad.
The question, though, is what happens when Perk comes back? I think Shaq, at this point in his career, is enough of a professional to realize that he will be coming off the bench.