Petrel 10 UL Women's Sleeping Bag

The Petrel UL with Muscovy down would be my pick - that higher fill power down is going to give you more warmth for weight, which matters when you're backpacking in. Since you're always cold I'd probably even consider going with 0°F rating instead of 10°F, especially if you're doing winter work in PNW For pads, definitely get something with high R-value since ground cold is usually what kills your warmth, not just air temp

That Feathered Friends has almost no feathers, all down. It is a spectacular bag with actual resale value if kept clean. A 650 fp has not nearly the warmth. I did this to myself - had a 600 fp bag and upgraded to a 800 fp quilt, it was night and day, wouldn't trade the quilt. Gave the bag to someone who was a fair weather camper who didn't backpack. I also won't really compress the quilt to the max, because stuffing and restuffing and overcompressing down wears it out faster. I got a good 15 years out of the quilts and had Rainy Pass rejuvenate them with 900 fp down. Now they're even warmer since I got a bit of overstuff added.

Western Mountaineering, Feathered Friends or Marmot for bags. Enlightened Equipment, Hammock Gear or Katabatic for backpacking quilts. Not cheap but quality and comfortable.

I've been a quilt user for years, but if I were looking for sleeping bags, I would go with either Western Mountaineering or Feathered Friends. You absolutely get what you pay for with either brand. Awesome quality.

I’m a woman who sleeps very cold and I use a 0F quilt on the JMT. I’ve never been too hot, even with nighttime temps in the 60s. Just stick a leg out or use it as a blanket. You have a weight penalty for bringing a warmer quilt, but down weighs less than extra clothes. A warmer quilt lets me take an alpha direct fleece instead of a puffy. If you enjoy spending a lot of time chilling with friends in camp though, you’ll want the puffy. I don’t think you’ll regret the 10F bag.

As is evident from the variety of responses here, unfortunately, this question can only really be answered by your own experience with particular gear. In general women's bags are rated for "ISO comfort" and men's bags are rated for "ISO limit" as women in general, need a 10F (the approx difference) warmer bag than men (ymmv) Note that ISO ratings assume a ASTM R5+ sleeping pad and a full base layer. Feathered Friends don't use the ISO rating system but they are generally regarded as making warmer bags. For the Sierras in summer a 20F bag with an ASTM ~R4 pad is the default recommendation for most. The combination will be too hot in some weather and possibly a little cold at 5am at altitude...at least that was my experience as a man using a 20F men's down bag and an X-lite (ASTM R4.2) next to snow in May (Desolation Wilderness) and July (around Forester Pass) in a high snow year (2019). Generally I found it too hot going to bed and cool at 5am. Wearing a mid weight base layer worked best to regulate the temp. My bag is not ISO rated but seems to have a fairly accurate limit based on my experience camping in the low 20s recently. (ie. I was a bit cold but I wasn't shivering and it didn't keep me awake. A 10F bag will be too warm for most in most places in summer, probably not warm enough for actual winter camping and only really good shoulder season. Since it will be quite a bit heavier to carry all summer when you are likely to do most of your backpacking I would only get a 10F women's bag if you really know you sleep cold. In many places in the US a 30F bag is too warm for Summer. If later you find you want to camp in the winter your first upgrade should be a warmer sleeping pad. An inexpensive way to do this is to combine your R4+ pad with a closed cell foam pad (eg Z-lite, Switchback ~ASTM R2+) as the combination is equivalent to ~R6+ pad. You can also use a synthetic (eg 40F-50F) quilt over the top of your down bag which can help keep the frost off the down. Your over-quilt should be light enough not to de-loft your main bag. You can try a thicker base layer and also bag liners which help some people but can be more trouble than they are worth. They can work well with quilts to minimize drafts.

Go for the 10° bag. You can't make a 20° bag warmer if needed, but you have options to make a 10° bag cooler - different clothing, partially zipped, arms or leg out of bag, etc. Plus, the 10° bag will be suitable for other trips, as you mentioned.

I thru hiked the JMT last Sept into Oct. Was concerned about shoulder season temps, so went with a 10 deg bag. It was overkill for 90% of my hike. You can always supplement with extra clothes, I'd go with the 20 deg bag.

Another vote for Feathered Friends or Western Mountaineering bags. They’re supposed to last for a decade or more and are very warm. I have a FF petrel and love it so much.

Absolutely great advice. I have a Feathered Friends Petrel and loooove it. I frequently camp at a National Park in my town. The campground is over 8000’ in elevation and is cold in the winter even though we live in a desert. My FF bag keeps me warm and toasty when other campers are freezing.