There's a lot of neat shell stuff out there, but in my opinion: choose permeable – whatever that looks like to you.
It was proven back in the late 80's that the "waterproof breathable" shells weren't working well on the cold weather bags, due to condensation and frosting occurring on the inside face of the sleeping bag's shell. Sadly, a lot of companies were seduced by the "GORE Zeitgeist", and made a lot of products without any thorough testing, especially toward vapor management. Even to this day, there still seems to be a lot of "build first, design second" mentality. Most items we use are probably best suited for specific climates. Why doesn't the industry tell us which ones they are best suited for?
Personally, I now believe that all sleeping bags should be as permeable as practical (as long as they keep the down in). Leave the "waterproof breathable" stuff to another system altogether. Even the lightest interior shell fabrics (Seven D, Nobul 1)all are somewhat wind resistant and have a slight dwr finish on them. To me, that is perfect.
Here's why I think the way I do about sleeping bags:
Inside a tent; having a built-in "waterproof breathable" shell is not only redundant, but the lack in permeability can raise the risk of condensation within the insulation, especially in a cold/humid environment (which is what most tents will eventually create in the right circumstance). A small amount dwr on the outer fabric is just fine, as long as it doesn't impact the permeability.
If one is cowboy camping directly under the stars, many experienced hikers can attest to the fact that (unless you literally sleep like the cowboys did – with wool and cotton blankets) it's very difficult to pinpoint if and where in their system that dewpoint might occur, especially on clear/cold/most evenings. Therefore, one should employ a separate system from the insulation (like a bivy) that can manage vapor transport and potential condensation. Either way, most bivy's will experience condensation at one time or another. Just have a good way to remove the water when it does.
I've never tried it, but I've wondered if a light cotton/fleece-like sheet might be in order, just under the outer shell of a bivy- just enough to absorb any condensation that might occur, or provide some much needed separation between the sleeping bag and the bivy shell.
(This is exactly how the old Bibler/Integral Designs Todd/Tegral tex worked.)
Finally, if your bag's insulation does ever gets wet, it will dry faster if both the inner and outer shell fabric is as permeable as can be. In the building world, it is easy to design a wall that keeps water out of it. It is difficult (and climate driven) to design a wall that both keeps water out, and properly lets water out of it when things get wet from the inside.
