As others have said, looks can play into your decision.
If you are considering urethane like the Mix, you will spend more, but you will probably have a longer lasting ball since urethane generally more durable. Urethane can hook more than plastic, but for a lot of people, the difference is negligible. In fact, if you purchase a Mix as your first spare ball, chances are you won't have any trouble.
I know some people that purchase a new spare ball almost yearly.