Here's the deal: Several years ago, the good folks at W.L. Gore introduced DryLoft, a "light" version of Gore-Tex designed to breathe very well but give insulated garments and sleeping bags a higher degree of water resistance than most other shell materials. The thinking is that even if your bag is in a tent, it's apt to get a little snow or rain on it as you open and close the tent door, drag in wet clothes, that sort of thing. Despite the $80 to $100 it added to the price, other fabric and bag makers followed Gore's lead. Conduit, for instance, is Mountain Hardwear's proprietary laminated material (it's waterproof, too). Pertex is a super-lightweight nylon material, a great bag shell but not inherently waterproof. Epic uses a silicon coating on fabric threads to create a breathable but water-resistant material.
Personally, I tend to think that these fabrics are solutions seeking a problem. Sure, getting moisture on a bag can dampen it and reduce its effectiveness, especially a down bag. But you'd really have to drench that puppy, not just get a few drops on the surface. Moreover, any material that keeps water out will also help keep it in. When sleeping, most people transpire up to two pints of water. If that water vapor wanders around the insulation waiting for the shell material to breathe it out, it may condense or freeze.
So, I think less is more, in this case. Without a shred of scientific study to support my belief, I think that polyester microfiber is the best shell material. It's light, downproof (meaning the fabric won't "leak" down), windproof, and inherently water resistant (polyester is extremely hydrophobic). And, it doesn't cost a ton. Western Mountaineering uses it in many of its bags. My next choice would be the new generation of superlight, breathable Pertex fabrics, which, although made from water-absorbing nylon, are treated for water repellency. Then, if you really find that getting a bag wet from the outside is a problem, choose any of the more water-repellent shells. They all work pretty much as advertised. I've had the most experience with DryLoft, and find it very good.