I mainly choose the NTCs because they are cheap and broadly available compared to tempco resistors. Kassutronics [1] and René Schmitz [2] have some background on the usage of the NTCs. PTCs should have the same effect when used in the op amps feedback path to change gain on temperature change (higher temperature = higher gain).
I don’t know about the LM13700 regarding the VCAs, they are even to expensive for my taste. I think I’m going for a simple op amp/transistor solution. I’m very interested in the JFET variant, I would definitely like to try that out!
[1] Kassutronics: VCO part 1: core
[2] Synth schematics--::--Exponential Convertors