Lets settle the bag type" question first. Id go with down for two reasons. One, it will be the most compactable bag by a considerable margin, particular in that 30-degree range you mention. And while no bag should be stuffed tightly as a drum and left for weeks at a time, a down bag will withstand that at least as well as a synthetic, and probably better.
With that in mind, several bags will work well. I like Marmots Arroyo ($250; marmot.com), a 30-degree bag with 800-fill down that weighs one pound, 11 ounces. Its an excellent buy in a high-performance bag. Its extremely compactable, particularly if used with the Sea to Summit eVent Compression Dry Sack ($30 in 8X18"; seatosummit.com), a tough, waterproof sack that uses a waterproof/breathable eVent membrane so air can get out but water cant get in.
I also like MontBells U.L.SS.Down Hugger #3 ($270; montbell.com), which also uses 800-fill but adds a series of patented elastic bands to the bag shell, so the bag gently hugs you as you sleep. This makes the bag warmer by reducing the tendency for cold air to get sucked into a bag when you flop around at night. I used one on a recent spring climbing trek and was warm as toast on a chilly (not 30), windy night. It weighs only one pound, seven ounces, and may be even more compactable than the Arroyo.
For a synthetic bag, take a look at Mountain Hardwears new UltraLamina 32 ($175; mountainheardwear.com). It uses a relatively new insulation called Thermic Micro, which promises to hold up well through many washings and compressions. Plus, Mountain Hardwear has welded the seams, not sewn them, so the bag can loft more readily. Very nice, and at one pound, 14 ounces, its a real lightweight in the synthetic realm.