can't compare to be honest. I bought 66789 as a new model, but I have returned the model as it is defective with the glue splash. Any that are re-sold will have to have the defects listed or they are repaired and sold on.
I do wonder what will happen to the returned models as I am in the process of returning 66621 but the whole item can't be re-sold as 66621 as the numbers on the side are now 66612.
Now if there was an option to buy defective ones, I would be interested in buying them for static display, TnT, DIT etc