I find it odd that the Rab Pertex Shield + tops out at 16K HH as found here http://us.rab.uk.com/products/mens-clothing/shell/volt-jacket.html
While the Montane Minimus Pertex Shield + has Fabric Waterproof Rating: 20,000 mm, Fabric Breathability Rating: 25,000 g/m² as found here http://www.backcountry.com/montane-minimus-jacket-mens

Oh well, I guess I am going to have to wait another year or two to see what pans out for the fabric.

One fabric weights 90gsm and the other is 54gsm. Also the Rab uses a stretch version which probably lowers the rating. One jacket weighs twice the amount as the other. Furthermore, do you have access to some Shield+? If not this would probably be better placed in the gear forum not MYOG.