Let me stick my oar into this old debate - Mark Kelly is exactly correct and Dertonarm is completely off base in his comment that " the inertia in a say 15 grams effective mass 12" tonearm with a given cartridge is always larger then in a 15 grams 9" tonearm".
The effective mass of a tonearm is in fact the inertia of the tonearm when it reacts to a deflecting force, which is what a warp is - that is what effective mass is BY DEFINITION. This is first year college physics. If the effective mass of a one tonearm is the same as the effective mass of another tonearm, it has EXACTLY the same inertia to a warp, regardless of its actual mass or physical dimensions.
The effective mass of a tonearm is in fact the inertia of the tonearm when it reacts to a deflecting force, which is what a warp is - that is what effective mass is BY DEFINITION. This is first year college physics. If the effective mass of a one tonearm is the same as the effective mass of another tonearm, it has EXACTLY the same inertia to a warp, regardless of its actual mass or physical dimensions.