You shold see a much harder backend reaction from the AF than from the AI, just due to the Vanguard upper flip cap core. I too would reccomend the AF, its a great ball for mediums, has many drilling options, and cn be used for multiple lines.
It may be a bit more drill sensitive than the Sahara, due to the stronger mass bias, but overall I think it should work better. It is true that the Sahara can work on mediums with a fairly strong drill, but you'll probably have to change the oob surface to something duller, and still you may be on the lighter side of oil handling capabilities.
Putting the pin on the negative side of the ball is not a huge concern. I have a Ravage (almost the same core as the Sahara) drilled with the mb beneath the thumb at 6" from my pap that really concerves energy. This layout CAN be drilled on a vanguard ball as the pin is on the midline not the negative side of the ball.
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