S-shaped arms are more accurate at the two null points of the arc the arm transcribes across the record surface, but it is less accurate at all other points. The straight arm is a little less spot on at the nulls but more closely algned throughout the rest of the arc.

Not quite true!

Let's say you've got two tonearms: one with an S-shaped armtube, and one with a straight armtube....

As long as both arms have the same effective length, both arms are mounted on the turntable with the same pivot to spindle distance, and both cartridges are aligned with the same headshell offset angle, then then two arms will trace the EXACT SAME path across the record surface! It can't be any other way!

Mike