Which luggage is more durable - that made of ballistic nylon or that made of 600 denier polyester?

The ballistic nylon will be a higher denier (thickness of the thread). Also will be shiny/glossy compared to the matte finish of polyester. Ballistic nylon will be much more durable and much less likey to tear or scuff. You can get higher denier polyester (900, 1200, 1800) which will be better for wear and tear. The price will reflect this however as 600d poly is low to mid price point whereas the others are higher.