Inspector FIA is the best job in the prospect of promotion and upgradation. it has more charm than any other department. in customs or preventive officer, you will never be promoted to BPS-17. But in FIA, the case is very different. You can even reach upto BPS-18 and BPS-19.
Example of Mr. Abdur Rehman Malik is in front of you as he was a simple inspector and then promoted to director.
So I will suggest you FIA.
I myself selected on open merit as Appraise/Valuation Officer but I didn't submitted my documents