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They are both the same family but just different types, Autumn Joy and Autumn Glory are very similar but the green one you mention is not as hardy and strong growing as the other two I mention.
It would be best IF you could send in a picture as there are many different types that could be described as yellow and they range from small low growing plants to the taller ones that grow as tall as your Autumn Joy, I would not throw out the green yellow one just yet as it may transpire that it is just needing different conditions to the others, UNLESS you just don't like it.
By the way these plants are so easy to take cuttings from and give you more plants if required, you just scrape away earth around the root areas and there will be small offshoots with roots attached, do this in early spring when new growth appears and select new plantlet's large enough to pot up and care for till large enough to set out and grow on as a plant for the borders.
Best of luck. WeeNel.