I don't really know much about green cheek conures but I do know that sun conures are the loudest parrots in that family lol. They are very beautiful though. When I was still in high school and living with my parents I had a Dusky Conure. His name is tex and my parents still have him. He is absolutely the sweetest parrot we have ever had. They are the smallest and quietest conures but no conure is really ever 'quiet'. I just love them!
We have a green cheeked conure,bought him as a baby.We hand fed him for a couple weeks till he could eat solid food.He has been a great pet,quite most of the time,but has his moments.Very smart, they are not great talkers.Real people birds.We have had him 9 years and not one problem.
Green Cheeked Conure - HANDS DOWN!!!! You won't regret it for a second. I can't find a thing wrong with them and I've kept parrots, parakeets, conures, etc for over 35 years. They are wonderful little creatures and beautiful to look at!
Sun's are very, very loud. Beyond loud! It's the major reason they are re-homed - often!
Sun conures are louder than Green Cheeks but are usually better pets and are less nippy. But Green cheeks are much cheaper and have quiet a few color mutations to pick from. I've handled both but conures in general turn me off.