I would tend to agree with Tonto. The G12-S costs 30 cr less than the G13-D balls, and they last longer. The meter is only a half ball faster, and the extra distance is swapped for better spin, but with any new ball, there's still a getting used to period.
Whatever you decide, stick with it until you reach the higher levels and can afford to switch to the high-end balls. You get accustomed to playing with certain properties, and what you would consider an upgrade only becomes a necessity to change the way you play the game.
Have fun regardless - Kelsey