My 2¢:
All else held equal, silicone is usually better. But I’ve softened my anti-PVC stance over the years, and I’ll explain why.
Regardless of insulation material, there are differences between individual brands and types. So first, let’s review the qualities we want in a test lead:
- flexibility (limpness) so you aren’t fighting the wire
- doesn’t “take a set” (doesn’t have “memory” of how it is bent)
- toughness, so the lead isn’t damaged in ordinary use
- safety, especially when used at mains voltage or higher
- long life
- resists tangling
- doesn’t pick up dust
- useful temperature range
- low electrical resistance/adequate cross section for the intended use
Some of these properties are contradictory. The biggest one is that limp wire is more liable to tangle.
So what influences those properties?
Flexibility is the result of two things: conductor stranding (huge number of ultra fine strands is desirable) and insulation material.
Both silicone and PVC can be equally soft — or stiff. I’ll take a high quality PVC over low quality silicone. Some silicone is designed only for its thermal properties, but without any consideration given to anything else. I’ve seen silicone cable with a silicone jacket so fragile that I could strip the 3x1.5mm2 jacket using my fingernail. (Really.) Another — a thin supposedly-test-lead-wire from Cal-Test had so much memory as to be useless for test leads. (And it really was the insulation, as it has memory even if all the copper is pulled out!!) I just got some thin Stäubli silicone wire today, and its insulation is so tough that none of my wire strippers can strip it — the insulation just stretches (to over 2x!) and bounces back. A lot of silicone is very “sticky”, grabbing onto any dust it comes near. That’s why it’s often dusted in talcum powder. If you clean it off, you may need to reapply talcum. Some silicone (like the wire Pomona sells and uses for Pomona and Fluke test leads) is a bit less limp, but has a “satiny” finish even without talc, and doesn’t tangle as much.
There are huge differences in PVC, too. It varies wildly in its initial softness, and varies wildly in how well it ages. Some plasticizers migrate or evaporate, making the wire stiffer and getting into nearby stuff; others are very stable and stay put. PVC is generally not a dust magnet. Toughness varies a lot.
As far as safety goes: you not only want the insulation to resist damage, but you want damage to be easily visible. This is why most high-quality test lead wire these days is double-layer, typically a white inside layer and the red or black outer layer, so that if the insulation gets damaged, you see the white and know to replace it. Test lead wire tends to have thicker insulation than regular wire.
Silicone resists heat damage (it won’t melt; at many hundreds of degrees, it will burn, leaving ash) and stays soft at very low temperatures. PVC will melt if you touch it to your soldering iron, and turn unusably stiff at winter temperatures.
With a gold-standard brand like Stäubli, both the silicone and PVC are top notch. I have used PVC of theirs that is decades old and still very, very soft. Both their PVC and silicone are extremely soft and tough. I can recommend either without reservations, but since the silicone is only slightly more expensive, I buy that. In top-quality test lead wire, a lot of the cost goes into the extremely fine stranding, so the insulation impacts the final price a bit less.
Fluke/Pomona silicone is excellent quality, but as others have said, some of their products had poor strain relief. Their cheapest PVC (like in TL70 probes) gets very stiff with age, but doesn’t get sticky.
Probe Master is outstanding; they only do silicone.
Another good brand is SKS/Hirschmann. Not quite as soft as Stäubli or Fluke/Pomona, especially the larger gauge PVC. Their ultra-small test leads (the ones with “DuPont” style connectors) use extremely soft PVC that is very nice.
In a nutshell, I’ve become more willing to accept very high quality PVC, which can actually be softer than silicone, at lower cost. In particular at small gauges (like 0.25mm2/24AWG), getting good silicone gets a lot harder, to the point that I’ve mostly moved back to PVC for that. Too many small silicone wires aren’t really designed as test lead wire and have fragile silicone with too much stiffness and memory.