This response is based on the information provided.  We suggest you discuss with your contracting team, program manager and/or legal department as appropriate. 

You need to have a Performance Work Statement (PWS), Statement of Work (SOW), and as required, a specification/performance specification.  This is how we convey what we need to the contractor.  The preference is to use performance-oriented direction and guidance.  The theory being we are contracting for expertise as well as product and/or service.  The choice your unit makes must be based on the specifics of your requirement.  If your requirement is directive in nature a SOW could be more appropriate in that you are not looking for "an innovative approach", "a better way".  You need specifically what you are asking for any number of reasons, e.g., consistency in inventory.  Whether using a PWS or SOW what we get from the contractor must be measurable, i.e., we can objectively show we got what we contracted for.  The type of contract does not, in and of itself, drive a PWS versus an SOW.  How we need to describe what we need drives the use of One versus the other and then given what we are acquiring and all aspects of the acquisition strategy then inform the appropriate contract that best encourages great contractor performance.