I`ve used them both in Remington hulls for 1 oz. The Claybuster 8100-12 is "Beefier" than the Downrange RT12 for lack of a better term.
However , as others have said the RT12 is "Slicker"which makes them slightly easier to load. Velocities are pretty close for all three of these pictured. When/if you switch to the RT12 , you will not notice any difference performance-wise.
I can see no difference in the amount of "Plastic Buildup" ( which isn`t much) in my gun whether I`m shooting Claybuster , Downrange , or Factory loads. From Left: CB 1100-12 , CB 8100-12 , Downrange RT12.
Regards , Pete