Back to the original question, I *think* both mods are important, but I don't know exactly because I have the SL1210 M5G which comes from the factory with upgraded wiring. I *have* noticed further (subtle) improvements by separating the interconnect leads and then loosely twisting them together. Based on that, I suspect that an upgrade from garden variety SL1200 tonearm wire to Cardas, Kimber, or whatever Kevin offers would have a welcome effect of more openness, more detail, more dynamics, airier presentation, improved soundstage, etc.
The fluid damper brings a different set of improvements to the party. I can't say which is more important, though I guess if you don't have good signal transfer from the cartridge, the rest of the tweaks won't be as noticeable either. Some critics say the stock SL1200 sounds dark. I would never say that of the SL1210 M5G, but hey, maybe it's the better tonearm wire.
OTOH, I disagree somewhat with Ed in that I found the fluid damper to be a noticeable improvement even with fairly low-priced cartridges, in my case, the Denon DL-160. It's not a dramatic night-and-day difference, and you have to make sure you don't overdamp, but once it's in, you get a greater sense of refinement, sweeter strings, clearer, more articulate percussion, more clarity and inner detail and bass detail. Its improved ability to track warps has made it a boon to this bargain-bin LP shopper.
Ed may well be right that these improvements manifest themselves more dramatically with upscale cartridges. I can't speak to that but I certainly felt that the trough was well worth it for the DL-160.
Given that great matchup, I bet the Sumiko headshell and trough would be killer with a Sumiko BPS EVO III or Blackbird.
