The master record cutting lathe moves in a stright line. Obviously the best method of recovering the groove information is to memic the lathe. A tangent tracking tonearm is expensive and extreamely fussy.
The reason older turntables (also todays less expensive) use 9 inch arms is: the overall size grows thus the cost and the turntable may not fit the average component rack.
The 12 inch radial arm is 3 inches longer than the 9 inch. This difference as shown in the diagram above (it may look like a small amount but when dealing in microns it is huge) relates to about 25% less tracking distortion. The longer the arm the shallower the arc.
One difference is the 12 inch arm's moment of inertia is slightly higher due to the extra length of the arm tube. But the arm tube is extremely light. The effective mass of the 12 inch is very close to the 9, the reason is effective mass is derived by a complicated equation that uses mass and inertia. If your turntable will accept a 12 inch arm the proformance difference will be very apparent. When you are ready to move to a 12 inch arm look at the PolyTable SUPER12 or the Merrill-Williams 101.3