Storm38,
If you are planning on drilling 3 new balls, one being for spares, and the other looks like a Virtual Gravity, I would recommend drilling a symmetrical ball for your third selection (a Thunder series ball, or a Fire line ball). If you are only using two "strike" balls, it is a very good idea to have at least one that is symmetrical in core shape. The Gravity compared to the Virtual are designed for two different volumes of oil, however the over all ball motion will be similar in shape on those two different conditions, and if that isn't the shape you need to carry, it will be a long night. However, giving yourself a different roll through the lane with two different core shapes, will give you more options to play the lanes differently.
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Tommy Gollick
Storm Regional/Pro Shop staff
Red Crown Pro Shop Harrisburg, PA
stormbowling.com