I use the SDS. Had considered the PLC (but never tried one) until I read that the SDS is significantly more effective, & also that the PLC radiates large amounts of interfering EMI into any nearby active componentry.
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I have both. As Bob mentioned, the PLC is electrically noisy and must be located carefully away from your signal path. The SDS doesn't have this problem. The PLC is a more primitive design with analog control knobs, while the SDS has push buttons and a digital read out. Having said all this, I haven't heard a big difference in sound quality, although the SDS has been praised by the magazine reviewers (better ears than mine, I guess). Good luck.
Michael Fremer did a review comparing the VPI SDS with the Walker motor drive controller. I don't recall what year or issue. Try looking it up on-line at Stereophile's Website. I recall that there was some useful info to be gleened from there.
I bought an SDS for my 19 Mk. III and find it to offer alot including variable voltage which allows me to lower the input voltage to the motor after startup. Plus, I have to admit, it looks much cooler than the Walker unit.