I already explained, but I'll do on here so everyone knows also hehe
A stiff rope halter/hackamore isn't the best thing to ride in because it will give unclear cues, and will delay your cue so the horse won't react as soon as the rider needs/wants.
Picture it this way...
There are two knots on the nose piece...those knots then you ask to stop are to put pressure on the nose in a certain place in order to stop the horse.
Your horse is galloping out of control, and heading for a cliff...your horse likes to fly hehe.. Anyways...you see the cliff and pull back on the reins to stop your horse, since your horse knows how to do sliding stops. How much pressure will you need to use to make that stiff nose band, put the knots in the correct spot and give the correct ammount of pressure to get your horse to stop before you fly to your doom?
A soft one is formed around the nose, so the reaction time is less and on your part you'll need less pressure (but to the horse it will be the same ammount of pressure)
If you are going to get a rope hackamore, I do suggest the softer rope material. I use the rope hackamore with Sonny and he listens better than in a bit normally.
I have the Parelli halter...just bought it, just used it for the first couple of times. I used to use a stiff rope halter. I immediately saw Sonny react quicker, better and I had to use less pressure to do a simple turn.