I would say a big difference in oil-handling ability. Look back to the RS-1 and the RS-P, which the Horizons respectively replaced. The Horizon pearl should get down the lane clean and then make a strong move on the back. The Mystic should get going earlier -- and make a firm, but very controllable, move to the pocket. Consider the pearl for lighter sides of medium and the particle-pearl for the heavier sides (dulled, possibly even heavier amounts). Together, they might form a decent league arsenal for the bowler not wanting to carry around a ton of equipment.
The particle-pearl really makes a difference. Some companies seem to come out with balls that do the same thing -- Roto does not.
Hope this helps.
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