Pretty much what Bob said.  You can't make a sweeping generalization as to which is better.  On one condition for a certain style of bowler, Assymetrics will be better, one a different condition for a different style, Symmetric is the way to go.
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The clock on the wall says 3 O'Clock...last call...for Alcohol!
"I love a martini -- but two at the most. Three I’m under the table; Four, I’m under the host."  Dorothy Parker

I guess my answer didn't come off quite right.  I was trying to say that I can't say which is better.  I bowl in 2 leagues, in one league I use only asymetric, the other I use only symetric, and I average right around the same.  When I switch, the scores drop.  The one I like better is the one that is working at the moment I need it.
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The clock on the wall says 3 O'Clock...last call...for Alcohol!
"I love a martini -- but two at the most. Three I’m under the table; Four, I’m under the host."  Dorothy Parker