I've just been in email contact with Joachim at Dietz, about a totally different question, and in the course of the correspondence have found one little thing that may add to the Dietz X3 vs. Massoth S debate (leaving aside for the moment the fact that you can't get S modules anywhere right now....
) - a sound unit connected via the SUSI bus will apparently NOT work at all under analogue (DC) power - it will only function on DCC control. Just something to bear in mind if you often take locos to open days on friends' layouts, if you go somewhere with DC only then a loco with a Massoth S will still produce sound while running (assuming the S is wired "conventionally", and not to the SUSI plug), but one with an X3 will be silent (loco will run, but no sound).
Having said that, any loco with a power buffer like a Powercap Micro fitted won't run at all on analogue either, but it's just another thing to take into consideration when making a decision which route to go down.
Jon.