Dear Peter,
we have to mate either tonearm with a cartridge with perfectly suitable compliance, - if done so, there will be no trade off in moving mass vs. effective length.
Given an ideal match with the compliance of the cartridge, we can assume almost identical conditions in terms of resonance frequency.
So the geometrical advantages of the 12"-tonearm will prevail.
The sonic advantages will be especially noteable on well recorded opera with comperatively large soundstage.
The sonic presentation of the soundstage will be much more stable and the positions of the various singers and their movements on stage will be more precise.
So - as I see it - there is no trade off between moving mass and geometrical advantage.
I do not want to call again and alone the example of the FR-tonearms (the FR-66 will always beat the FR-64s with any given FR-7 system), so lets go to Ortofon instead:
- do mount a SPU in a 9" Ortofon and in a 12" Ortofon tonearm. Both do have effective moving mass together with the SPU which will result in resonance frequency very close to each other (but it will be lower in the 12" Ortofon). Given exact mount and alignment to the same geometry (Baerwald, Loefgren, Bauer, Stevenson - no matter which, as long as its the same for both), the 12" will prevail in terms of size of soundstage presented, ease of tone and naturalness.
All these features are related to lesser derivation from the zero-error-curve - because the 12" tonearm does come closer to the theoretical ideal tangential.