According to my understanding, the essential issue is that your parents must convince the VO that they have stong ties in China and they will go back to China ON TIME. Therefore, you should emphasize on TIES. Whether you or your spouse invites, whether you have GC or not are not that important. If you parents have job and real estate in China, that will be much better.
Sometimes the reaction to VO is also very important. Each VO has interviewed hundreds of applicants. They are good at determining whether the answer are the true or are just for getting the visa.
Don't use those agents. They only provide tips for visa interview. They can't guarantee the success.
Good luck!