I have some opinions.....
I do not like nylon. Rubs my horses, stretches when it is wet, stiffens up when it gets old. I do not even like nylon halters.
I HATE Indian leather. TRASH. Whatever you do, DO NOT buy harness made of this. It just snaps when it gets tired of living, and YOU are suddenly adrift in the carriage, and your horse is free....and that is the flowery picture.... sometimes, you die.
I love leather harness, especially with brass hardware, but there is a TON of care required with leather harness. Wiping it down, cleaning, oiling, storing it in a controlled environment so it does not mildew, or become too dry, polishing.
I do not care for Zilco brand Biothane. It is quite expensive, and does not hold up well. I use it for training, because it is still serviceable, but looks HORRIBLE.
Regular biothane, either beta(matte finish) or regular(shiny finish) is my everyday harness. It washes on the horse fro day to day, or in the washing machine for a fun show.