Peter,
If you read the SME web pages on the two arms you will notice that this sentenced is suppressed in the SME V-12 features
"Internal constrained layer damps minute residual vibration leaving the tone-arm acoustically inert. Fine machined cartridge platform, enamel free to avoid interface resonance. "
IMHO this can suggest that there are considerable differences between the two arms other than length and consequently tracking angle. Theoretically we could even consider that the SME V-12 is a "less perfect" tonearm considering SME objectives enumerated in the SME original features. Surely perhaps your preferences are not the same as those of SME V designers ...
Interesting points, Francisco. I have no way of knowing the [sonic?] preferences of the SME V designers, so I can not comment on that one. Should we assume that their preferences changed between the designs of the two arms beyond the desire to reduce tracking error by increasing the length of the arm? It is surely possible that their preferences did not change. Perhaps founder ARA was the final arbiter of the sound of these arms. I think he was involved in the design of the 12" 312S arm just before he died and I understand that he wanted to develop the 30/12 table to support a 12" version of the V arm. The 20/12 was developed for the 312S arm, or visa versa.
The V-12 was released after ARA died, and perhaps the omission of that line in the list of features was an accident by those who put together the web pages. Do you know if the armtube's internal damping and cartridge platform (headshell?) are in fact different between the two arms? I would find it surprising to learn that they omitted these features in their flagship arm if they thought it made the arm sound better.