RST has been talked about as very similar to the Physix, but a rounder motion
The Rubicons are supposed to be like strong symmetrics with a weight hole
In a 6 ball or smaller arsenal, I dont know if there'd be enough separation to warrant 2 of the Physix/RST/Rubicon Solid. I think you could have both the Halo Pearl and Rubicon uc2, but that's mainly based on my opinion of the Centruum core being very forward rolling