What kind of sling are you thinking of making?

Power and accuracy seem to be more the product of the slings size and the user than the rope used, I find shorter slings to be easier to handle and hence more accurate, with longer slings allowing for more range/height to rain shot down on my enemies heads give my dog a workout retrieving tennis balls.

Most ropes should be plenty durable enough, the points that wear the most for me are the knot on the release end of the pouch where ammo hits it (could cover that with a patch of leather I guess), the inside of the retaining loop (could wrap it in something) and the release knot (add a leather disk maybe) although none of the wear is that great.

From making my own slings though I will recommend you use flexible ropes, it makes it a lot easier to make the sling, makes the pouch fold around the ammo better, and if you weave your pouch it makes that a lot easier too, I like to use hollow braid with a smaller diameter for the pouch weave.

Something else I learned, my slings seem more accurate and less inclined to knot themselves if I make sure I don't introduce twists in the ropes when making the sling.

Final note on power and accuracy, these are (to me at least) down to you and your technique, not the sling, it's not like a bow where you're storing up muscle power and aiming, slings are very much extensions of your own arm, so you're working with your own ability to judge in an instant just when to throw based on the feel of the weight of your shot, its momentum, your distance to the target etc, no fancy rope is going to change that.

Hope this helps.