First of all, no, theres not a huge difference in weight between a zero-degree bag and one rated to 15 or soreally, only four to six ounces. Its better to get what really works for you.
Second of all, you of course do SOME layering, I assume. Its prudent to at least wear long underwear as that helps keep the bag cleaner. And these days there are so many good light layersdown sweaters from Patagonia, Mont-Bell, Marmot, or others sell for around $150 and offer fantastic warmth in a tiny packagethat you could wear one to bed and hardly know it, getting double-duty out of a piece you can wear during the day as well.
Next, which bag? You will not go wrong with either manufacturer. Both Western Mountaineering (westernmountaineering.com) and Feathered Friends (featheredfriends.com) make what are perhaps the best bags in the country. Western makes both zero-degree and 15-degree bags. In the former, theres the Kodiak Super MF ($460 in the six-foot length), which has the MicroLite XP shell you mention. Thats a very fine-weave polyester shell that breathes well yet is down-proof and highly water-resistant. In the 15-degree range, the Apache Super MF ($375 in the six-foot length) has similar construction but of course a bit less down. Its a wonderful bag.
Ive been skeptical about Feathered Friends use of EPIC, a siliconized fabric, but I had a long talk with one of their guys a few days ago, and he insisted that it works wonderfully in sleeping bags. So Ill take that at face value, plus there is the fact that Feathered Friends has been doing this so long they ought to know their business by now. Their Murre is a zero-degree womens bag that goes for $459 in the medium size with the EPIC shell. The Petrel is a ten-degree bag that is $409 with EPIC.
I wish I could give you a firm recommendation one way or the other, but I cant. I really like polyester shells, such as what Western uses. But I havent used a bag with EPIC so cant speak to it directly. My feeling is that the EPIC-shelled bag might resist water a little better, at the cost of slightly more moisture buildup inside. Not enough to notice, though. Plus the womens-specific construction (slightly different bag shape and insulation distribution) in Feathered Friends may be a factor for you to consider.
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