If you're going to use it as a ground cloth, get something cheap like the plastic tarps - it'll end up being useless as a rain shelter in a few trips. It would make no sense to spend a bunch of money on cuben and put it on the ground, walk on it, and potentially poke holes in it. While it's fairly durable, it's not going to withstand sharp rocks forever.

I would get a tarp and then get some 2mm painter's drop cloth to use as a groundsheet. Or polycryo, which is the same as the shrink wrap you can add to window panes - not expensive in fairly large rolls from hardware stores, cut to suit your needs. If you have a dedicated ground cloth it doesn't matter if it has a few holes, you can use it longer, and when it's beyond use you can recycle it and cut out a new one.