Both, as they're not the same sort of thing. The Sisters is a very weird sort of multimode-meets-formant arrangement, great for overall spectral reworking. But the Optomix is actually a pair of lowpass gates with a summing mixer, which means you can take your Sisters outputs on into that and apply the percussive Buchla-esque 'plooks' with the Optomixes LPGs. Put a normal Sisters output on one LPG and the "Centre" on another, and you can tandem your sound back and forth percussively in patterns between "clean" and "FMed" filter responses. Fun stuff!