I havent used either (and I kinda doubt that most of the people here have either since they are both RC papers) but I have Canson's swatch book and the Satin is more of a 'brushed' surface (think difference between regular stainless steel and 'brushed'). The reflections are a bit 'softer' and more diffuse w/ the Satin. The Lustre being slightly 'harder'. It's fairly subtle.
If I were to try a lighting metaphor, think softbox vs. beauty dish (w/ a sock diffuser). The Satin is more softbox-y and the Lustre somewhat 'harder'