What lasts longer under the elements: polyester rope or galvanized wire?
Galvanized rope won't rust quickly at all the galvinization prevents rust from forming. Do you see any chain link fencing in your area it is similar but a larger wire size than the strands in 1/4 rope but this could provide an idea. Poly rope will break down from the UV light in sunlight it starts to "chalk" at first then the outer strands start breaking leaving sharp little wiskers. I hang electrical wires on galvanized rope all the time but would never use any kind of other rope as it won't last as long as the galvanized. Note if after you assemble your clamps you have scratched the wire you can use a Zink rich paint or a spray can of cold galvanizing spray paint on the hardware and it will outlast almost anything.