I emailed S&S about this is and here is their response "Most of the powedered sweeteners that we had looked at over the years had contained prominent amounts of Citric Acid which can bring some people out of ketosis, so for this reason we advise to use the tablet form sweetners as the ones we had looked at did not contain any."
I've found a powdered sweeetner that doesn't contain citric acid so I'm happy to continue to use that as it's more versatile. But I guess it's up to your personal preference.