I recently acquired a Bozeman Mountain Works Vapr Bivy. I haven't had it out in wet weather yet, so, in order to test the water resistance of the Pertex Quantum top, I ran a rather simple experiment: I laid the bivy out on a table, placed a paper towel inside it, and poured a small pool of water on top. After about an hour, the paper towel was already beginning to get damp. When I checked it this morning (10 hours later), the water had completely soaked through. Ron at MLD reported that, in a similar experiment, Momentum had no soak through at all. Moreover, he's found Momentum to be just as breathable as Pertex Quantum.
I doubt that the increased water resistance of Momentum would be all that noticeable in typical tarp camping conditions, where Quantum is more than up to the task of keeping rain spray at bay (as evidenced by Ryan's Lost Coast trip). Nonetheless, given the two options, are there any good reasons not to choose Momentum? Ron obviously doesn't think so, but does anyone else have an opinion? Does Quantum have any other properties that offset its lesser water resistance?