Ace,
I have both balls and love them equally. With my speed, and with the UI polished, they can both be used on mediums. The shot shape will be different, however. The UI starts up a whole lot sooner and seems to be best suited for playing straighter lines. The AI starts further in the midlane and reacts more like a typical pearl, long and strong. Personally, I think a completely dominant arsenal could be made out of just these two balls. The question about which one you want really depends on the type of shot that you like to play. If you want to be able to go in deeper and swing it, the AI will probably be the better choice, not that the UI can't be used in that way. If you like playing up the boards, the UI is tough to beat. Just my .02. Good luck and good bowling to you.
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I can't bowl 300, but I can bench 330 : )