Hey gamma,
I'd go with a Machine litely polished. I played with this cover from sanded to polished and have left it polished to get some separation from my Artillery and it works great on med to med dry conditions (for the avg tweener).
In regards to the pattern, the Machine offers a much stronger mass bias differential compared to the Robo which will have a different effect on it's overall reaction dependent upon its placement. I'd highly recommend to discuss with your ball driller there to see what he rececommends since he/she may know your game better. Eric Martinez (Track Staffer) is also at one of the shops I believe from seeing here on BR, maybe he can be of some help to you with his knowledge of the local bowling centers and their oiling procedures.
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Rick Leong - Ten Pins Pro Shop
Track Intl. - Amateur/Pro Shop Staff
Vise Inserts Staff
www.Trackbowling.comwww.startabowlingrevolution.comSee profile for Track Ball videos
*El Presidente of the Track Revolution