If you are 100% applying for Endo, then you are better off doing the GPR/ AEGD. It will strengthen your app more than a 1 year associateship. Could you get in after a one year associateship, sure. But succeeding in a GPR/ AEGD, obtaining a good LOR and learning from an Endodontist (who most likely will be staffed) will just make you a more predictable applicant.