Silicone is meant for high temperature but has high friction. The high friction limits movement in the wire bundle, increasing stress on the inner and outer wires, making the bundle act like a big fat wire. And silicone is also very flexibile which actually hurts because the flexibility means that the stresses are carried more by the copper. PTFE wire is less flexible, and generally has thicker sheathing which acts as a better strain relief, but has lower friction as well.
In my experience, PTFE sheathing is thinner than silicone, allowing you to stuff more of them in the same cable chain.