Like all tools of the trade there are personal preferences. I have seen all types of lines used for tracking, rope, climbing rope, leather, and all types of webbing. They all have some good points and some bad. I personally stay away from any form of rope as it has a tendency to burn your hands as it slides through. Yet it is strong, easy to obtain and inexpensive.
Leather is costly but has the most natural feel and spring to it.
The down side is that when you buy a 20 foot line after a number of tracks you soon have a 30 foot line and as time wears on a 40 foot line which can break as the line pulls thinner and thinner. It is strong, inexpensive and long lasting. Be sure to buy a quality webbing which is supple so it won't be too hard on your hands. Line length is something there will always be a lot of discussion about. Personally I feel that 20 feet is about the right length.