To go along with Nathan's post, with the Gravity Shift being an Asymmetrical Pearl and the 2D being Symmetrical Pearl you're looking at two different reaction points depending your style of play. Traditionally with myself and customers of mine who have both balls, the Gravity Shift is a longer skid/flip reaction type. Down Lane and sharper breakpoint but very controllable. Great for some of the longer flatter patterns The 2D, albeit pearl, creates more of a stronger mid-lane roll. Great for medium length and some of the shorter patterns.
Both covers are very tunable, although I personally think the 2D can go much lower than the Gravity Shift grit wise. They both a spot in the bag so for me to say "exchange" would be hard.
If you were to look for a ball to switch to when the Dimension is too much, the 2D would be the better option of the two, but getting out of that line would be a better option, like going to a Hy-Road or even a Street Rod.
I'm sure you'll be happy with whichever decision you make though, good luck.
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Justin Buford
Orlando, FL
2009 Storm Bowling Staff Member
Rich Gibson's Pro Shop at Boardwalk Bowl Orlando, FL
http://www.myspace.com/wildcat300http://www.stormbowling.com