You should finish processing the 175, if you got a Student Visa now you'd still have to apply for a PR visa after your studies and starting all the expense and paperwork over again. With a 175 you're done when it's all finished and free to be in AU as a student or whatever you wish. If the processing has been > 1 yr ask your Immigration case officer if they need any other info and when it's reasonable to expect an update about your visa.