Don't auditors have to follow both AICPA and PCAOB?
AICPA is for private companies. PCAOB establishes standards for public companies.
no you can pick which one you want
It is not optional. You have to comply with PCAOB standards, regardless of whether AICPA standards are applicable or not. It is not optional
AICPA standards are for all practitioners who audit or work for private (not SEC registered) companies. PCAOB are for practitioners who work for or audit publicly traded companies, per the Sarbanes-Oxley Act these standards are set for SEC registrants. There is a choice of sorts, but they are both mandated if you work with both. It really depends on the company the practitioner is engaged to audit/work for. Although I agree with Nehal, there is a bit of a choice where the client is concerned.