First, a few words about Feathered Friends. They're based here in the Seattle area, and I own several of their products, so I'm very familiar with them. They're sort of the Mercedes-Benz of down gear - some makers turn out stuff that's as good, but nobody really beats them in any meaningful fashion. Beautiful stuff: high quality down, great shell materials, excellent construction. The Tern is a superb bag - rated to -10 degrees, it's a true winter bag for mountaineers or winter backpackers, yet it packs down to a remarkably small volume, and weighs just over three pounds. There are 20-degree bags that don't weigh that little! It also has a fat draft tube along the zipper, a hood roll that fits over your forehead, and a neck collar to reduce the "bellows effect" caused when warm air is pushed out as your turn around in the bag. Note, however, that the Tern is cut quite trimly to save weight - if you like to sleep with your feet splayed, or tend to be claustrophobic, this may not be the bag for you. And it's not cheap$460 to $490, depending on the shell chosen.
Mountain Hardwear's Universe SL also is a fine bag. It's rated to 0 degrees, and at that weighs about six ounces more than the Tern. It also uses slightly lesser down than the Tern 600-fill down, versus the 750 stuff in a Feathered Friends bag. So that adds an ounce or two to the weight, and theoretically means the Universe will have a slightly shorter life span. But you'll be plenty warm.
I'm not ecstatic about the shell choices from either maker in this case, however. Feathered Friends gives you a choice of Epic Nextec, which uses a silicon-coated fiber, or PTFE Lite, an out-of-patent Gore-Tex formulation. Mountain Hardwear uses Conduit, a proprietary waterproof-breathable fabric. I think ALL of them are too much for a cold-weather bag. When it's 0, it's not raining, and in fact is apt to be very dry. So why a waterproof or highly water-resistant shell? That just cuts breathability, and may lead to frost buildup inside the bag. So here's a third choice: Western Mountaineering's Puma Super Microfiber ($410 in regular size). It's a -10 bag that like the Tern uses 750-fill down. A little heavier because it's a little wider, but still very compact. And it uses a polyester microfiber, which is light, tough, water-resistant and very breathablethe best cold weather shell material, if you ask me.