Mix balls are made of urethane. There used to be a big difference between urethane and plastic (actually polyester) 25 years ago. With today's resins, being the main coverstock material, there is a lot less difference than there used to be.
Urethane and plastic will have larger differences in ball reaction with 2 different circumstances: one, with drier lane conditions, or light oil, urethanes will hook more than plastic; two, bowlers who are rev dominant will find urethane hooking for than for bowlers who are matched in ball speed and rev rate or for speed dominant bowlers.
Aside: urethane or plastic balls when used with more dynamic cores (larger RG differentials) vs, the more traditional pancake cores, will have more overall ball reaction but still very little compared to resin coverstocks.
