I tend to use one or the other. the lack of audio in on the model series means if I’m using one with a 2nd unit I naturally gravitate to something that has audio in such as for me octatrack or digitone, digitakt etc so I don’t need a mixer.
lack of audio in on the models is my main gripe with em, personally.
They work great together, the M:C generates gorgeous kicks, percussion, chords, bass and harsh noise. The M:S can bet set to act as the backbone to the M:C because it provides samples for expanding the tonal palette beyond the unfiltered FM sounds of the M:C.
If you like the M:S enough to get another one, go for it. Nobody can help with that decision, it’s too personal with a lot of variables.