Geoffrey Owens of Helius Designs makes a 9", 10", and 12" version of his excellent Omega arm. It is his opinion that the 10" provides the best balance between the advantages and disadvantages of all lengths. 12" arms, while providing significantly smaller tangency error than do 9", have far greater moment-of-inertia mass, generally not a good thing. Geoffrey feels the 10" arm’s tangency error advantage over the 9" is worth the only-slightly greater m-o-i of the arm, a justifiable trade-off. But the m-o-i penalty of the 12" in relation to the 10" is greater than it’s lower tangency error advantage over the 10", an inadvisable compromise.
The introduction of the Trans-Fi Terminator air-bearing linear-tracking arm has changed the landscape of that design. Go to the T-F website for details. Perhaps the greatest tonearm value right now---just over $1000!