I personally like the soft feel of the VInyl slugs. I only use the pink ones, because as Mike said, color affects the texture of the slug. I tried urethane ones and they always felt too hard and slippery to me, but it really comes down to personal preference. My only advice is that if you find a color of Vinyl that feels good to you, stick with it for every ball.
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MainePBA
"Power tends to corrupt absolute power corrupts absolutely." Lord Acton