Even for someone working in the industry, where we are trained on this kind of thing, it is an intense amount of info.
Extremely Brief History (based entirely on what I have been taught):
Gore-tex has been the leader in the waterproof-breathable business since W.L. Gore produced the first membrane way back in the olden days.
...That's it. Seriously, I said it was brief. Anyway the more interesting stuff has happened more recently. (Warning: this may get a little technical...ish)
So as more membranes entered the game from more manufacturers, all claiming outstanding waterproofing and outstanding breathability, some method had to be devised to test these membranes. It was, and it was used to test some membranes. The results of these tests are available, summarized in from Natick Soldier Research (the US army has good reasons to want excellent waterproof-breathables). Yay! They are publically available over the intertubes by searching: "Breathability Comparison of Commercial Outerwear".
It's a pretty comprehensive test that rates pretty much all of the leading membranes in the industry. Some membranes show a humidity-dependent curve, the others (scoring better in general) showed a constant level of breathability across humidity differences. It rates Expanded PTFE as the most breathable, followed by eVent, Schoeller dryskin (softshell), and Entrant GII. My understanding is that Gore-Tex Pro Shell is based around expanded PTFE. The test also tested Gore's XCR and their original membrane, which fared much worse (only reaching moderate breathability levels at the highest humidity difference).
And yet, in real-world tests? The magical internets say, among other things, that eVent is more breathable than Pro-Shell.
So what's the deal? I'd argue that pretty much every membrane out there is going to work, otherwise they wouldn't make it into clothing. Pretty good, that you read this far without a definitive answer eh? Awesome.
Today I was trying to find a more recent test and I came across an OLD paper about a pretty interestingly real-world way to test these fabrics. It simulated rain falling on the fabric as the test occurred, unlike the other tests usually cited by manufacturers. The most intriguing part of the test was the effect that the rain had on the breathability: cooling the air next to the fabric, raising the humidity on the outside, and actually causing vapour to condense on the inside of the fabric! (a common complaint with most membranes).
Very interesting the difference rain makes!
Current Lusts: Patagonia R3 Jacket, Patagonia Speed Ascent Jacket, Cloudveil Koven Plus Jacket