Here are my two cents. Yes the cia is great for internal audit but if you ever venture out of it the cia is just disregarded compared to the Cpa. Even in ia the majority of individuals in a Fortune 500 audit shop will have their cpa. Most cpas look down on it and only see it as a secondary cert. I have both by the way.
in my opinion cpa holds up better even if you only want to be in auditing, both external and internal. cia is scrictly internal auditing and most say is inferior to cpa. plus cpa has more options should you want to change fields, which is becoming more common these days