9", 10" or 12" tonearms

Indeed - a great thread which will nicely illustrate the inner truth in Syntax' most recent comment here.
Highly recommended to everyone looking for insight into "the game"..

The longer tonearm results in less derivation from the tangential zero tracking error line. Sonically it gives a wider soundstage with more stable individual positioning. Furthermore you can expect to hear less distortion in climax' and critical high frequency passages. All these are direct results of a more " stable aligned" position of the polished area of the stylus seen in the horizontal plane.
All the above are only a relative advantage between tonearms of the same design (VPI, DaVinci, SME, FR etc.) but available in different total length.
Its similar to sailing boats - length gives smoother run.....
If possible ( many TTs do not offer an option to mount tonearms with a total length in excess of 10") I would always go for a 12" tonearm. The geometrical advantages do in general (with good designs..) more than make up for increased effective moving mass and (sometimes...) small decrease in stiffness.
If you look around you will notice that its always the 12" versions of vintage tonearms (SAEC, FR, SME, Ortofon, DaVinci etc.) which do command the high prices on the second hand market. They were more expensive then their shorter brothers too when new, but they tend to preserve their value much more stable and often do increase over time ( due to their rarity too).
The "kings" of pivot tonearm designs are all 12" versions.

Of course you are right Audiofeil, tonearms like the SME 3012 (any version), FR-66s, FR-66fx, Ikeda IT-407, Ortofon 309, EMT, DaVinci 12, SAEC 506, SAEC 8000, AC-4400, Schroeder 11.5" are all 12" (or slightly more... or less) but do range rather low, both in their manufacturers order of importance/quality/design/build quality/implementation or price-list.
Thus reflecting through price policy their level of quality and performance.
That too is the reason why they go so cheap in the used market and why there is no demand for them either.
Thank you for setting this straight.
With another 29 years experience in high-end audio I will certainly getting it.

Lets not forget the fact, that the majority of all plinth turntables can NOT accommodate tonearms with an effective length greater than 10" (even the VPI 12.x and Kuzma 4Point tonearms do use a 9-10" mounting distance with off-center bearing location to move around this fact).
Thus any tonearm designer with a remote interest in sales will think twice ( and set aside the whole idea... ) about launching a 12" tonearm.
And it was that way ever since.
Custom audio born tonearms in 12" were always introduced as "off-springs" of their older 9-10" brothers.
All exceptions worth mention were broadcast-clones.
Aside from the points you mentioned - which most circle around less stiffness and increased dynamic moving mass (effective...) - this is one paramount "practical no-go" for 12" tonearms.
But I guess we all can agree, that in terms of geometry the 12" have a natural advantage in terms of tracking arc aspects. The increased mass is no question - well maybe one day we will see the re-rise of low compliance cartridges. Isamu Ikeda just revised his cartridges to lower the compliance......
Yes - in tonearm design it is all about careful choosing and weighting the needed compromises against each other.
The fact that we see many cartridges today with quite heavy body weight but quite high compliance ( certainly NOT a good mixture... it considerably increases the effective mass of ANY pivot tonearm ) doesn't make the game any less demanding ....