My statement that "nylon is stronger than polyester" may be misleading, or at least oversimplified. There are different ways of measuring fabric strength: tensile strength, tear strength, burst strength, and more. Also strength when wet, when heated, etc., not to mention the effect of UV radiation.

Fabric strength (whichever kind of "strength" we're discussing) is also affected by the weave, coating if any, thread count, and probably other factors, too.

So, I don't have a good answer for you. If your application is a tarp for backpacking, both nylon and polyester should work well.
The only tarp I've used is a homemade "Ray-Way" style tarp made of 1.3oz. silnylon, which worked well.
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Always remember that you are absolutely unique, just like everybody else. -Margaret Mead