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Modulation sources, definitely hands down.
DivKid did a great video on the Vector Space. If I had the space, I'd get one. It should give you some interesting results when combining modulation sources. The Batumi is fine. But I sound like a broken record on the forums when I say to get the Poti along with it. You won't have to fuss with the jumpers on the back if you want LFO shapes like triangles, saws, etc.

If you'll be working with volts per octave, you may want to get a buffered mult and possibly an adder (for accurately summing CV).

Here's the video for the Vector Space.