It's not really about letting water through as much as it is water seeping through when you make contact with the inside surface. Sil will allow water to pass-through if you press against the inside of the tarp. Spinn, being the glorious material that it is, does not. My understanding is that Cuben material possesses the same attribute.
Ah, like the good old canvas tents used to do when you touched them.
I do still remember that lovely smell
Guess it's going to be a spinn for me.