depends on what you really want to do. If want to continue stay in
SPF for life, getting the degree is just to improve your CEP (whatever that is called in
SPF), so quite honestly what you study doesn't matter, just choose something you like &
SPF can approve.
if you are intending to study some totally difference course with intention to run road after 5 years, you better think twice. by that time you will be competing with all the fresh grads in the job market with only a part time degree, too high existing pay, and stigma of irrelevant exp & culture in civil service