I like the looks of the Montbell Super Stretch bags (check out Backcountry.com’s lineup) which have elastic stitching to provide a roomier night’s sleep. While I haven’t actually slept in one, I do know that Montbell makes generally excellent lightweight gear, though South Bay types have an in-born fondness for Western Mountaineering’s bags (check out Backcountry.com’s assortment) which are built in San Jose. Everybody wants one; start saving for next year if you can’t afford one this year.
I went synthetic with the first bag I ever owned (the Marmot Pounder Plus), but if I had to do it over I’d go with down. Synthetic bags are a bit better in rainy environments, but if you’re in the Sierra in the summertime you’ll almost never encounter rain at night. You will have to dodge afternoon thunderstorms, but if you keep your bag stowed properly there’s little chance you’ll get it wet. Down requires a bit more care, but it generally lasts decades, while synthetic bags start losing their loft after a few nights out on the trail. My Pounder Plus, rated at 25F, barely kept me warm in a night in the high 30s the last time I took it out, and that was after less than a dozen bag nights.
If you’re a newbie, Backpackgeartest.org is the best place to start scouting for a bag. The reviews are from actual users (vs. magazine writers), which requires a bit of patience but provides ample details on how gear performs in the real world. You might also appreciate this thread at Backpacker magazine, which weighs the merits of rectangular bags vs. mummy bags, which are a bit more difficult to sleep in because they’re so much unlike your bed at home. If you go this route, buying a hat to keep your head warm is pretty much mandatory.
Whatever you do, make sure your bag fits before you take it out on the trail. Ideally you’d test the fit in a store, but that’s no substitute for actually sleeping in it. One night on the back porch should tell you all you need to know.
Which bag do you like best? Let loose in the comments.
Related link: How to wash your down sleeping bag.