I would do the sound generator. Here's why:
The sequencer is not a bad sequencer. However, it's redundant with powers that the Force has, and in particular the Force can do a whole bunch of sequencing with minimal drama so the SQ64 doesn't really buy you a whole lot more.
On the other hand, the Force is a little more limited in terms of sound channels. Also, while there are all sorts of sounds in it, and you can add plenty more (one way or another), adding synthesis power to the Force is generally where you will get more bang for your buck, if only in terms of CPU power saved.
Also, while I'm a fan of the Volca series in general, I do think that it would help to stop and ask yourself the question that NearTao proposes: what do you want to achieve, and how will other devices help you to achieve this? For example, would a Volca drum machine be your best match? For not a lot more money, you could even get something like a Novation Circuit, which would get you a couple of voices of percussion as well as melodic tones. It's also a very nice lap sketchpad, if that's part of your workflow (or if you'd like it to be).
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