SME iV.Vi or SME V for SME 20 turntable?

The damping trough IS an option if you buy the arm separately. However, I dont think the IV.Vi is sonically any different than an SME V -- it's an identical mechanism -- EXCEPT for two things: the internal wiring, and the aforementioned damping trough.

The IV.Vi was designed with MC cartridges in mind, which due to their usually low compliance, don't really require damping.

The internal wiring is actually a coated ribbon instead of the vdH 150M used in the V arm. Initially, SME used Magnan for their OEM ribbon, but now makes their own. According to SME, the ribbon does two things: offers less resistance to movement of the bearings, and less electrical resistance to the tiny signal produced by MC cartridges. This supposedly allows MC cartridges to "be all they can be".

One disadvantage, if you don't get the optional damping trough, is that you also don't get the tonearm height adjustment mechanism. SME IV and V arms are difficult enough to height adjust, so not having this feature is a real PITA, IMO ;--)
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Nsgarch is correct. I completely forgot about that tonearm height adjustment mechanism. And the arm IS somewhat of a pain in the butt to adjust, as I've realized when dealing with long play records of varying degrees of thickness. However, I do not agree with his contention that the IV.Vi and V are sonically the same. To my ears at least (As mentioned in my first response, I have owned both arms.), the IV.Vi has slightly more top end information, and perhaps just a tad less fullness in the bass. This combination of factors produces what I hear as more of an even balance throughout the sound spectrum. All said, however, both arms are top notch, and are my favorites among all arms I have owned, including the Zeta, an earlier version of the Triplanar, the Alphason, and several of Harry Weisfeld's VPI tonearms.

Opus, both arms are indeed physically identical in terms of parts, materials, etc. So if you are hearing a difference, it has to be due to the internal wiring differences. And it stands to reason that the wiring should produce sonic differences, especially with the very low output MC cartriges (.3 mV +/-), otherwise, why would SME bother making this special version?
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Your point is wll taken, Nsgarch, and you are right. My comments did not reflect your observation from the standpoint of separating the matter of wire differences from the tonearm itself. My apologies for the oversight.

The 'tonearm height adjuster' of the V - which is the VTA/SRA adjuster - is, imo, worth the cost difference. The adjuster screw has a fine thread which allows much more accurate and smaller incremental changes than raising the post by hand.

Fwiw, the IV.Vi (not the IV) as spec'd by Sumiko is described by them as having ".50 guage Magnan type Vi" internal wiring. Maybe their site info is dated. I speculate the arm cable to phono input will have as much an impact on sonics as the internal wire.

Tim