These days, when it comes to a synthetic bag you have basically two choices: Polarguard, which comes in three flavors (HV, 3D, and the new Delta) or Primaloft. There are some others — Quallofil and Thermolite, to name two — but they’re on the margins. Polarguard is a fine synthetic insulation. It’s been around for some time, has a reputation for good durability, and is very affordable. A classic Polarguard bag is The North Face’s 0-degree Snowshoe ($209), which uses Polarguard Delta. Weight is not bad at three pounds, eight ounces. The knock on synthetic bags is that they aren’t as compressible as down bags, but Delta is pretty soft, and you can always buy a compression stuff sack to help squeeze it down.
Primaloft is good stuff, too. It’s softer than Polarguard, so it drapes better over the body and to me feels more comfortable. It’s very warm, and seems to retain a little more insulating capability when very wet than Polarguard. But it’s a little trickier to manufacture a Primaloft bag, so they cost a bit more. Integral Designs’ North Twin is a fine Primaloft bag, rated to ten degrees, priced at $220. Like the Snowshoe, it weighs three pounds, eight ounces. From a practical standpoint, I doubt the two bags “real” temperature rating is all that different, as Integral Designs is quite conservative with its ratings. But either bag would work well for you.