The primary difference between VCAs and FETs lie in their transistor design.The VCA's transistors are placed in an IC and react to the input signal by modulating it with a control voltage. FET compressors work with the electric field in a way that more incoming voltage results in lowered resistance in the drain circuit, leading to gain reduction. In the FET design, the electrical charges are added to the voltage to calculate the variation in gain. Since the FETs are working on the principle of a gate, we have the voltage pushing through the gate, which causes a reduction in the resistance. So in an FET, the higher the voltage passing through the gate, lesser the resistance. Lesser the resistance, more the gain reduction applied. However, VCAs and FETs are quite similar in application due to their fast response. They can both be used for fast moving transients and dynamically rich material. But, a FET will add character to your sound and colour it by squashing your transients. A VCA, on the other hand, will make your track sound smooth, clean, and transparent by smothering your transients. So if you're looking to add character, grit, and colour on isolated instruments like acoustic guitars, rock vocals, or snares, its better to go for a FET compressor. But if you're looking for a lot of control over a group of instruments in a drum bus or a master bus, with a wide range of response times, you're better off going for a VCA.