IIRC, (and it's likely that I don't as I'm going back 25 years or so) the "S" arguement is that the angle keeps the cart in alignment for a higher percentage of the time as a record plays through, and the "Straight Arm" arguement is that it's lower in mass and therefore tracks irrecularities better.
I was considering a Stanton purchase, and they have versions of some of their tables with straight or curved arms. I looked on their site for some info in choosing but could not find anything on the subject!
