I had a PLC and upgraded to the SDS. It was a very small difference, although I've heard others state the difference was quite substantial. Most people are very sensitive to speed fluctuations. My guess is that in some environments under some conditions the SDS will perform significantly better than the PLC. In my room the power is already very well conditioned which might have had something to do with the small change I noticed.
Got an old VPI PLC......
I have one of the old VPI power line conditioners. My SAMA has a IEC plug on it so I made a 8 ft. cord so the PLC won't induce any noise. Thing is this thing is dead on accurate. It's one of the newer ones that lowers the voltage after about 5 seconds. I've heard talk about how a SDS or the Walker will improve the sound of my 'table (HW19 MKIV). How can that be if these devices just control motor speed? I mean the SDS is almost a grand. Will I get a thousand dollars worth of sound improvment?