Hi
The MS often takes 1 year to complete while the MBA takes 2 years. So the 1st question is how much time you could dedicate for study?
The second question is how many exp you gain during 6-year working (i.e you only work in Procurement or you have other exp in other parts of SCM - plan, source, make, deliver?- what's your current position - executive or manager?) General, the MS will provide you the in-deep knowledge in your major while the MBA will teach you more about the management skill - hence, if you want to study MBA, you should have some basic about the management - related to your work experience.
In short, if you want to study in-deep about the SCM and for specialist position, study MS. If you are holding the manager position and want to develop more about the management skills, study MBA in which focuses on SCM.
Thanks.