Ropes. And good quality hemp ropes are great, silk seems a bit excessive.
A good Dominant can do so many more things with ropes than just tie your hands up.
There are lots of factors involved that affect the decision, but that said, steel more often than silk.
Handcuffs usually - they are a turn on. But ... silk rope or a tie mmmm nice too lol
Why limit yourselves, ladies? Both have so many wonderful uses and applications! As do scarves, robe belts, etc etc.! ;)
None* of the above.
Leather for me.
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Edit:typo fixed*
Silk rope is ok but I prefer leather.