Polarguard is made with what are called long-staple fibers, meaning the threads that comprise a sheet of Polarguard are very long. When sewn into a sleeping bag, these long threads give the stuff its durability. Because of this it technically doesn't wear out, though over time can separate and form gaps. The biggest downside of this fill material is that long-staple fibers are fairly stiff. The Polarguard folks have been working to iron out this kink, but if you lie on your back in a Polarguard-filled bag, you'll notice that the top of the bag tends to form something of a triangle. A down-filled bag, on the other hand, will sag around your body, creating a warmer, more comfortable bag.
This stiffness is especially noticeable, however, in clothing. Which is why not too many garments are made of Polarguard. Here, softer insulators such as PrimaLoft really shine. PrimaLoft uses shorter fibers than Polarguard and is an attempt to mimic the softness of down. And it really does a pretty good job in this respect, this being why I rate it for clothing (and even in gloves) and also in sleeping bags. The supposed knock on PrimaLoft is that it isn't as durable as Polarguard, but I haven't had any problems with the PrimaLoft bags I own. It's also extremely water-repellent, due to its hydrophobic polyester threads.
PrimaLoft Sport is what's used most frequently now; it's an improved version of earlier PrimaLoft PL1 and PL2. Integral Designs uses PrimaLoft Sport in its Dolomitti Jacket ($160; www.integraldesigns.com), a really nice layering or standalone piece for cold, damp weather.
So which to use? For a bag, either Polarguard Delta or PrimaLoft Sport is fine. In the Delta, the ol' reliable choice has long been The North Face's Cat's Meow ($169; www.thenorthface.com), which is rated to 20 degrees Fahrenheit, or about minus six Celsius. Mountain Hardwear's 2nd Dimension ($175; www.mountainhardwear.com) also uses Delta and is rated to 15 degrees, nearly minus ten degrees Celsius.
For clothing, get something with PrimaLoft. The Dolomitti is fine. So, too, is the Arc'teryx Fission SV (www.arcteryx.com), which puts Gore-Tex over the PrimaLoft for a really weatherproof design. Mind you, this sheath of weather armor is also a rather pricey $499.
For more sleeping bags, check out Outside's 2004 Buyer's Guide