When it comes to choosing between weight and warmth, I always go for cozy. There's nothing better after a long day on the trail than to be able to zip into something more comfortable, especially if it comes with a hood and sleeves. Your Rab pick is right on. The 750+ fill goose-down Microlight Alpine Jacket ($215; us.rab.uk.com) comes with a Pertex laminate coating that makes it tougher than your average nylon, allowing for jabs with the tent pole or close encounters with sharp kindling. Best of all, if you're lucky enough to get some hot summer nights, the 340-gram jacket compresses into the size of a baseball and can live at the bottom of your pack until you want to take it out and use it as a pillow.
An equally warm alternative to down that you can use as a mid or outer layer is Outdoor Research's Neoplume Hoody ($150; outdoorresearch.com). I was skeptical that such a lightweight piece could keep me warm in the waning days of late August in Alaska, but I put it through the ringer on a soggy four-day trip down the Yukon River, and the PrimaLoft Eco insulation kept me warm, mostly because the rain slid right off.