Im half inclined to answer, neither." By far, my preferred lightweight-but-warm piece these days is a down sweater. It packs up to the size of a softball, weighs next to nothing, and is comfy down to freezingperfect for early autumn on the Wonderland Trail. And if it rains, well, just throw a shell over it. Patagonia makes a wonderful down sweater called, not surprisingly, the Patagonia Down Sweater for $200. MontBells Light Down Jacket ($150) is very similar.
So Id take that, a shell, and probably a mid-weight long-sleeve T-shirt (Marmot Midweight Crew, $40), and a light short-sleeve tee. That would give you several combinations to wear when hiking or loafing around camp.
If youre set on fleece, get something 200-weight, as that offers the most versatility. Mountain Hardwears Monkey Man Jacket ($140) is an excellent piececomfortable, warm, and stretchy. On the bargain side, L.L. Beans Super Fleece 200 ($60) is plenty of mid-weight jacket.
I hope the hike goes well for you. I know the park lost a lot of trails last winter, but I think the route is in pretty good shape now. Early September is the ideal time of year to do it.
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