Of the three you mention, the right one will to some extent be based on the camping you expect to be doing. The Cross Mountain ($129; www.bigagnes.com) or Wiggy's Nautilus ($110; www.wiggys.com) both have synthetic insulation, which is better if you camp in soggy conditions (although if it's sub-zero that's sort of a non-issue). The Cross Mountain is the lighter of the two at two pounds six ounces; the Wiggy's is three pounds but unlike the Big Agnes bag has insulation all the way around. Big Agnes makes bags that have no insulation on the "bottom" but that have a slot for a sleeping pad. That integration is certainly a big plus for those who indulge in nocturnal chase-the-sleeping-pad wriggling.
The Feathered Friends Great Auk has down insulation, so it's lighter (two pounds) than the other two, and more comfortable as a standalone summer bag because down is softer than synthetic insulation. But down can dampen out more readily than synthetics. And it's more expensive: $270 in the regular length (www.featheredfriends.com). Which is rather dear. Mountain Hardwear makes a more reasonably priced overbag, called the Down Upgrade, that sells for $160 (www.mountainhardwear.com).
If weight isn't an issue, I'd lean toward the Nautilus as its Lamilite insulation is extremely durable and the bag comes with a lifetime warranty—any problems, Wiggy's will fix it or replace it. I like the Great Auk because it makes such a great summer bag, as well as being a very good winter overbag. But price there is an issue. Overall, then, the Cross Mountain seems to have the most in its favor—good price, decent weight, good performance (which extends to summer use on its own, too).
For more sleeping-bag reviews and accessories, check out Outside's 2004 Buyer's Guide.