Synthetic rope is
I tend to agree that if you don't plan to be a frequent user of the winch, I'd stick with the wire rope unless you really just want to reduce the weight you are carrying around. I didn't follow my own advice of course and went synthetics so
As for the old/new cable - I'm holding on to my 7/16" stuff that came on my M15K in hopes that we can use it at one of our adopted trails. It was never used but did ride around on the winch for a couple years.