Can you please let me know to become more competitive for specialty programs (particularly Endo), would work experience is more important or a year of GPR/AGED?

I personally recommend actually starting dental school first and then going to the endo dept and talking to them about it.

Since the question presumes that a person, for whatever reason, does not go into an endo program directly after dental school, I would say doing a GPR/AEGD vs an associateship etc. would be the better choice. Completing a post grad GPR or AEGD program that is structured and still somewhat academic in nature would likley find more favor in the eyes of endo program directors vs a notation of having worked for Dr. GP at Anytown Clinic for a year or two.